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

[Dr. David Alan Gilbert]

* Huanyu ZHAI (huanyu.zhai@outlook.com) 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 following patch in the kernel:
> 
> https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1452368.html
> 
> 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().

I just spotted this when trying to clear out my mail folder;
this isn't a 'trivial'!

I'm not sure if it's right or not, but I've cc'd in Paolo and Marcelo as
x86 maintainers.

Dave

> 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)
> --
> 2.27.0
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK