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x86/pat: Simplify the PAT programming protocol

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The programming protocol for the PAT MSR follows the MTRR programming
protocol. However, this protocol is cumbersome and requires disabling
caching (CR0.CD=1), which is not possible on some platforms.

Specifically, a TDX guest is not allowed to set CR0.CD. It triggers
a #VE exception.

It turns out that the requirement to follow the MTRR programming
protocol for PAT programming is unnecessarily strict. The new Intel
Software Developer Manual (http://www.intel.com/sdm) (December 2023)
relaxes this requirement, please refer to the section titled
"Programming the PAT" for more information.

In short, this section provides an alternative PAT update sequence which
doesn't need to disable caches around the PAT update but only to flush
those caches and TLBs.

The AMD documentation does not link PAT programming to MTRR and is there
fore, fine too.

The kernel only needs to flush the TLB after updating the PAT MSR. The
set_memory code already takes care of flushing the TLB and cache when
changing the memory type of a page.

  [ bp: Expand commit message. ]

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20240124130650.496056-1-kirill.shutemov@linux.intel.com
This commit is contained in:
Kirill A. Shutemov 2024-01-24 15:06:50 +02:00 committed by Borislav Petkov (AMD)
parent b401b62175
commit ffc92cf3db
2 changed files with 7 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
arch/x86/kernel/cpu/cacheinfo.c
View File

@ -1118,15 +1118,16 @@ static void cache_cpu_init(void)
unsigned long flags; unsigned long flags;
local_irq_save(flags); local_irq_save(flags);
cache_disable();
if (memory_caching_control & CACHE_MTRR) if (memory_caching_control & CACHE_MTRR) {
cache_disable();
mtrr_generic_set_state(); mtrr_generic_set_state();
cache_enable();
}
if (memory_caching_control & CACHE_PAT) if (memory_caching_control & CACHE_PAT)
pat_cpu_init(); pat_cpu_init();
cache_enable();
local_irq_restore(flags); local_irq_restore(flags);
} }

9
arch/x86/mm/pat/memtype.c
View File

@ -240,6 +240,8 @@ void pat_cpu_init(void)
} }
wrmsrl(MSR_IA32_CR_PAT, pat_msr_val); wrmsrl(MSR_IA32_CR_PAT, pat_msr_val);
__flush_tlb_all();
} }
/** /**
@ -296,13 +298,8 @@ void __init pat_bp_init(void)
/* /*
* Xen PV doesn't allow to set PAT MSR, but all cache modes are * Xen PV doesn't allow to set PAT MSR, but all cache modes are
* supported. * supported.
* When running as TDX guest setting the PAT MSR won't work either
* due to the requirement to set CR0.CD when doing so. Rely on
* firmware to have set the PAT MSR correctly.
*/ */
if (pat_disabled || if (pat_disabled || cpu_feature_enabled(X86_FEATURE_XENPV)) {
cpu_feature_enabled(X86_FEATURE_XENPV) ||
cpu_feature_enabled(X86_FEATURE_TDX_GUEST)) {
init_cache_modes(pat_msr_val); init_cache_modes(pat_msr_val);
return; return;
} }