53666664a3
Explicitly tell the compiler that VMXOFF modifies flags (like all VMX
instructions), and mark memory as clobbered since VMXOFF must not be
reordered and also may have memory side effects (though the kernel
really shouldn't be accessing the root VMCS anyways).
Practically speaking, adding the clobbers is most likely a nop; the
primary motivation is to properly document VMXOFF's behavior.
For the flags clobber, both Clang and GCC automatically mark flags as
clobbered; this is noted in commit 4b1e54786e48 ("KVM/x86: Use assembly
instruction mnemonics instead of .byte streams"), which intentionally
removed the previous clobber. But, neither Clang nor GCC documents
this behavior, and there's no downside to including the clobber.
For the memory clobber, the RFLAGS.IF and CR4.VMXE manipulations that
immediately follow VMXOFF have compiler barriers of their own, i.e.
VMXOFF can't get reordered after clearing CR4.VMXE, which is really
what's of interest.
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David P. Reed <dpreed@deepplum.com>
[sean: rewrote changelog, dropped comment adjustments]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20201231002702.2223707-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.