Re: [PATCH] target/i386: Clear xsave pkru bit when KVM XCR0 not support

[Dongli Zhang]

Hi Yuchen,

On 5/17/23 03:55, Yuchen wrote:
> Migrating guest from Intel new CPU (as Gold 6230) to old CPU (as
> E5-2650 v4) will pause on the destination host. Because old CPU
> not support xsave pkru feature, and KVM KVM_SET_XSAVE ioctl
> return EINVAL.
> 
> This kernel commit introduces the problem:
> ea4d6938d4c0 x86/fpu: Replace KVMs home brewed FPU copy from user

This kernel commit issue should be resolved by the below kernel commit.

x86/kvm/fpu: Limit guest user_xfeatures to supported bits of XCR0

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ad856280ddea3401e1f5060ef20e6de9f6122c76

Since the old target server does not support pkru, I assume the VM's cpu type
should not support pkru. Therefore, the pkru should never be migrated away from
source server.

Dongli Zhang

> 
> Signed-off-by: YuChen <yu.chen@h3c.com>
> ---
> target/i386/xsave_helper.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
> 
> diff --git a/target/i386/xsave_helper.c b/target/i386/xsave_helper.c
> index 996e9f3bfe..64e2b969fe 100644
> --- a/target/i386/xsave_helper.c
> +++ b/target/i386/xsave_helper.c
> @@ -6,6 +6,8 @@
>  #include "cpu.h"
> +static bool has_xsave_pkru;
> +
> void x86_cpu_xsave_all_areas(X86CPU *cpu, void *buf, uint32_t buflen)
> {
>      CPUX86State *env = &cpu->env;
> @@ -47,6 +49,9 @@ void x86_cpu_xsave_all_areas(X86CPU *cpu, void *buf, 
> uint32_t buflen)
>          stq_p(xmm + 8, env->xmm_regs[i].ZMM_Q(1));
>      }
> +    if (!has_xsave_pkru) {
> +        env->xstate_bv &= ~XSTATE_PKRU_MASK;
> +    }
>      header->xstate_bv = env->xstate_bv;
>      e = &x86_ext_save_areas[XSTATE_YMM_BIT];
> @@ -181,6 +186,9 @@ void x86_cpu_xrstor_all_areas(X86CPU *cpu, const void 
> *buf, uint32_t buflen)
>          env->xmm_regs[i].ZMM_Q(1) = ldq_p(xmm + 8);
>      }
> +    if (xsave->header.xstate_bv & XSTATE_PKRU_MASK) {
> +        has_xsave_pkru = true;
> +    }
>      env->xstate_bv = header->xstate_bv;
>      e = &x86_ext_save_areas[XSTATE_YMM_BIT];
> --
> 2.34.1
> -------------------------------------------------------------------------------------------------------------------------------------
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