e54debe657
* Explicitly make some hardware constants part of the uabi -----BEGIN PGP SIGNATURE----- iQIzBAABCgAdFiEEV76QKkVc4xCGURexaDWVMHDJkrAFAmRKkrkACgkQaDWVMHDJ krDtDBAAhWbKRK1rJJsz2GuliF3/f/cZwcNxGG+QGrYBl2F2ilOrmVwNYME2TvHD qQJHm8pU7vnDpnkZspqE0OoB6fbSa5qH3RfFhBFRziJFgN9mY0F0IJZeuH/EvJ/0 7gkRMA3Fs41EESbAWhUTakvC6u3L06SUpUH2W8ixAcawZu+g/FksDXxE+eVVPZaQ Ztw17j6/m8W9bZ17HtyWK2vAepPlJhuXFPSAk7ox09ACwkqWAHO0/3RPcbc8HUZV lDyYeDhRELG1pai14GhTixRcgkdn4nnnNDmn13xpuwkpOh7FeZL/SoDmXtJ71CrJ I1YM1t9aB4ze2WDOo3mSKzU4efspGzAgIH26u19NQTmEp/9ppS+RaifXpt0r1yir ygOXkgk8l2qZPxryyL9ROU6b9cnPzsP9k3mWTtNJiJrx0CL73lWkA5KORb/Ezdnj kXAjTd4nUeCQJz+7PsnuvGqsT8/Dk1ugnHTu6Bn66U0hV0MNcx5G5m5HehDQBUmb TllHGJSGt/1AXIfBZ1p7GSrgCaq3NTzWNmcFxHS3bpC/pyGwszmdDBIS/pODfBfp 0nG9cG8mte1KkhqjkSYTLtgarQEijs1NWrVnTUogg1kqtlvqZr8Zxun51YAW9Jt5 zCGoB6W7EWVfJZBMHmVX7a4g21650mgte3YoAAyAwMJFtZG14ng= =GlmS -----END PGP SIGNATURE----- Merge tag 'x86_fpu_for_6.4' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip Pull x86 fpu updates from Dave Hansen: "There's no _actual_ kernel functionality here. This expands the documentation around AMX support including some code examples. The example code also exposed the fact that hardware architecture constants as part of the ABI, but there's no easy place that they get defined for apps. Adding them to a uabi header will eventually make life easier for consumers of the ABI. Summary: - Improve AMX documentation along with example code - Explicitly make some hardware constants part of the uabi" * tag 'x86_fpu_for_6.4' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: Documentation/x86: Explain the state component permission for guests Documentation/x86: Add the AMX enabling example x86/arch_prctl: Add AMX feature numbers as ABI constants Documentation/x86: Explain the purpose for dynamic features
175 lines
6.5 KiB
ReStructuredText
175 lines
6.5 KiB
ReStructuredText
Using XSTATE features in user space applications
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================================================
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The x86 architecture supports floating-point extensions which are
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enumerated via CPUID. Applications consult CPUID and use XGETBV to
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evaluate which features have been enabled by the kernel XCR0.
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Up to AVX-512 and PKRU states, these features are automatically enabled by
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the kernel if available. Features like AMX TILE_DATA (XSTATE component 18)
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are enabled by XCR0 as well, but the first use of related instruction is
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trapped by the kernel because by default the required large XSTATE buffers
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are not allocated automatically.
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The purpose for dynamic features
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--------------------------------
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Legacy userspace libraries often have hard-coded, static sizes for
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alternate signal stacks, often using MINSIGSTKSZ which is typically 2KB.
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That stack must be able to store at *least* the signal frame that the
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kernel sets up before jumping into the signal handler. That signal frame
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must include an XSAVE buffer defined by the CPU.
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However, that means that the size of signal stacks is dynamic, not static,
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because different CPUs have differently-sized XSAVE buffers. A compiled-in
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size of 2KB with existing applications is too small for new CPU features
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like AMX. Instead of universally requiring larger stack, with the dynamic
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enabling, the kernel can enforce userspace applications to have
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properly-sized altstacks.
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Using dynamically enabled XSTATE features in user space applications
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--------------------------------------------------------------------
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The kernel provides an arch_prctl(2) based mechanism for applications to
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request the usage of such features. The arch_prctl(2) options related to
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this are:
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-ARCH_GET_XCOMP_SUPP
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arch_prctl(ARCH_GET_XCOMP_SUPP, &features);
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ARCH_GET_XCOMP_SUPP stores the supported features in userspace storage of
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type uint64_t. The second argument is a pointer to that storage.
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-ARCH_GET_XCOMP_PERM
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arch_prctl(ARCH_GET_XCOMP_PERM, &features);
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ARCH_GET_XCOMP_PERM stores the features for which the userspace process
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has permission in userspace storage of type uint64_t. The second argument
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is a pointer to that storage.
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-ARCH_REQ_XCOMP_PERM
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arch_prctl(ARCH_REQ_XCOMP_PERM, feature_nr);
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ARCH_REQ_XCOMP_PERM allows to request permission for a dynamically enabled
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feature or a feature set. A feature set can be mapped to a facility, e.g.
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AMX, and can require one or more XSTATE components to be enabled.
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The feature argument is the number of the highest XSTATE component which
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is required for a facility to work.
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When requesting permission for a feature, the kernel checks the
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availability. The kernel ensures that sigaltstacks in the process's tasks
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are large enough to accommodate the resulting large signal frame. It
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enforces this both during ARCH_REQ_XCOMP_SUPP and during any subsequent
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sigaltstack(2) calls. If an installed sigaltstack is smaller than the
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resulting sigframe size, ARCH_REQ_XCOMP_SUPP results in -ENOSUPP. Also,
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sigaltstack(2) results in -ENOMEM if the requested altstack is too small
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for the permitted features.
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Permission, when granted, is valid per process. Permissions are inherited
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on fork(2) and cleared on exec(3).
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The first use of an instruction related to a dynamically enabled feature is
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trapped by the kernel. The trap handler checks whether the process has
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permission to use the feature. If the process has no permission then the
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kernel sends SIGILL to the application. If the process has permission then
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the handler allocates a larger xstate buffer for the task so the large
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state can be context switched. In the unlikely cases that the allocation
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fails, the kernel sends SIGSEGV.
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AMX TILE_DATA enabling example
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Below is the example of how userspace applications enable
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TILE_DATA dynamically:
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1. The application first needs to query the kernel for AMX
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support::
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#include <asm/prctl.h>
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#include <sys/syscall.h>
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#include <stdio.h>
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#include <unistd.h>
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#ifndef ARCH_GET_XCOMP_SUPP
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#define ARCH_GET_XCOMP_SUPP 0x1021
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#endif
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#ifndef ARCH_XCOMP_TILECFG
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#define ARCH_XCOMP_TILECFG 17
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#endif
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#ifndef ARCH_XCOMP_TILEDATA
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#define ARCH_XCOMP_TILEDATA 18
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#endif
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#define MASK_XCOMP_TILE ((1 << ARCH_XCOMP_TILECFG) | \
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(1 << ARCH_XCOMP_TILEDATA))
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unsigned long features;
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long rc;
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...
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rc = syscall(SYS_arch_prctl, ARCH_GET_XCOMP_SUPP, &features);
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if (!rc && (features & MASK_XCOMP_TILE) == MASK_XCOMP_TILE)
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printf("AMX is available.\n");
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2. After that, determining support for AMX, an application must
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explicitly ask permission to use it::
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#ifndef ARCH_REQ_XCOMP_PERM
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#define ARCH_REQ_XCOMP_PERM 0x1023
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#endif
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...
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rc = syscall(SYS_arch_prctl, ARCH_REQ_XCOMP_PERM, ARCH_XCOMP_TILEDATA);
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if (!rc)
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printf("AMX is ready for use.\n");
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Note this example does not include the sigaltstack preparation.
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Dynamic features in signal frames
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---------------------------------
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Dynamcally enabled features are not written to the signal frame upon signal
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entry if the feature is in its initial configuration. This differs from
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non-dynamic features which are always written regardless of their
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configuration. Signal handlers can examine the XSAVE buffer's XSTATE_BV
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field to determine if a features was written.
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Dynamic features for virtual machines
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-------------------------------------
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The permission for the guest state component needs to be managed separately
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from the host, as they are exclusive to each other. A coupled of options
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are extended to control the guest permission:
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-ARCH_GET_XCOMP_GUEST_PERM
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arch_prctl(ARCH_GET_XCOMP_GUEST_PERM, &features);
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ARCH_GET_XCOMP_GUEST_PERM is a variant of ARCH_GET_XCOMP_PERM. So it
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provides the same semantics and functionality but for the guest
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components.
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-ARCH_REQ_XCOMP_GUEST_PERM
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arch_prctl(ARCH_REQ_XCOMP_GUEST_PERM, feature_nr);
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ARCH_REQ_XCOMP_GUEST_PERM is a variant of ARCH_REQ_XCOMP_PERM. It has the
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same semantics for the guest permission. While providing a similar
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functionality, this comes with a constraint. Permission is frozen when the
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first VCPU is created. Any attempt to change permission after that point
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is going to be rejected. So, the permission has to be requested before the
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first VCPU creation.
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Note that some VMMs may have already established a set of supported state
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components. These options are not presumed to support any particular VMM.
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