[PATCH] x86/pat: Simplifying the PAT programming protocol

[PATCH] x86/pat: Simplifying the PAT programming protocol

[Kirill A. Shutemov]

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.

Turned out the requirement to follow the MTRR programming protocol for
PAT programming is unnecessarily strict. The new Intel Software
Developer Manual[1] (December 2023) relaxes this requirement. Please
refer to the section titled "Programming the PAT" for more information.

The AMD documentation does not link PAT programming to MTRR.

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.

[1] http://www.intel.com/sdm

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Juergen Gross <jgross@suse.com>
---
 arch/x86/kernel/cpu/cacheinfo.c | 7 ++++---
 arch/x86/mm/pat/memtype.c       | 9 +++------
 2 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/cpu/cacheinfo.c b/arch/x86/kernel/cpu/cacheinfo.c
index c131c412db89..78afad50a7c0 100644
--- a/arch/x86/kernel/cpu/cacheinfo.c
+++ b/arch/x86/kernel/cpu/cacheinfo.c
@@ -1118,15 +1118,16 @@ static void cache_cpu_init(void)
 	unsigned long 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();
+		cache_enable();
+	}
 
 	if (memory_caching_control & CACHE_PAT)
 		pat_cpu_init();
 
-	cache_enable();
 	local_irq_restore(flags);
 }
 
diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
index de10800cd4dd..7cac1b56c5e7 100644
--- a/arch/x86/mm/pat/memtype.c
+++ b/arch/x86/mm/pat/memtype.c
@@ -240,6 +240,8 @@ void pat_cpu_init(void)
 	}
 
 	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
 	 * 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 ||
-	    cpu_feature_enabled(X86_FEATURE_XENPV) ||
-	    cpu_feature_enabled(X86_FEATURE_TDX_GUEST)) {
+	if (pat_disabled || cpu_feature_enabled(X86_FEATURE_XENPV)) {
 		init_cache_modes(pat_msr_val);
 		return;
 	}
-- 
2.41.0

Re: [PATCH] x86/pat: Simplifying the PAT programming protocol

[Juergen Gross]

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On 28.12.23 13:58, Kirill A. Shutemov wrote:
> 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.
> 
> Turned out the requirement to follow the MTRR programming protocol for
> PAT programming is unnecessarily strict. The new Intel Software
> Developer Manual[1] (December 2023) relaxes this requirement. Please
> refer to the section titled "Programming the PAT" for more information.
> 
> The AMD documentation does not link PAT programming to MTRR.
> 
> 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.
> 
> [1] http://www.intel.com/sdm
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Juergen Gross <jgross@suse.com>

Assuming other vendors' cpus (e.g. Hygon) are fine with this:

Reviewed-by: Juergen Gross <jgross@suse.com>


Juergen


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