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Re: [PATCH] cpu/i386: update xsave components after CPUID filtering

From: Paolo Bonzini
Subject: Re: [PATCH] cpu/i386: update xsave components after CPUID filtering
Date: Fri, 24 Feb 2023 09:29:31 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.1

On 11/23/22 19:17, Huanyu ZHAI wrote:
Subject: [PATCH] cpu/i386: update xsave components after CPUID filtering

On i386 platform, CPUID data are setup through three consecutive steps: CPU model definition, expansion and filtering.

XSAVE components are enabled during the expansion stage, by checking if they are enabled in CPUID. However, it is still

probable that some XSAVE features will be enabled/disabled during the filtering stage and the XSAVE components left unchanged.

Inconsistency between XSAVE features and enabled XSAVE components can lead to problems on some Linux guests in the absence of

The patch is correct, but I think you can also remove the existing call to x86_cpu_enable_xsave_components(). Can you test that?



the following patch in the kernel:


A simple case to reproduce this problem is to start a SUSE 12 SP3 guest with cpu model set to Skylake-Server:

$ qemu-system-x86_64 -cpu Skylake-Server ...

In the SUSE 12 SP3 guest, one can observe that PKRU will be enabled without Intel PKU's presence.

That's because on platform with Skylake-Server cpus, Intel PKU is disabled during x86_cpu_filter_features(),

but the XSAVE PKRU bit was enabled by x86_cpu_expand_features().

Signed-off-by: Huanyu ZHAI zhaihuanyu@huawei.com <mailto:zhaihuanyu@huawei.com>

Signed-off-by: Xin Wang wangxinxin.wang@huawei.com <mailto:wangxinxin.wang@huawei.com>


target/i386/cpu.c | 3 +++

1 file changed, 3 insertions(+)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c

index 22b681ca37..2ee574cf05 100644

--- a/target/i386/cpu.c

+++ b/target/i386/cpu.c

@@ -6362,6 +6362,9 @@ static void x86_cpu_filter_features(X86CPU *cpu, bool verbose)

             mark_unavailable_features(cpu, FEAT_7_0_EBX, CPUID_7_0_EBX_INTEL_PT, prefix);




+    /* Update XSAVE components again based on the filtered CPU feature flags */

+    x86_cpu_enable_xsave_components(cpu);


static void x86_cpu_hyperv_realize(X86CPU *cpu)



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