linux-kernel - Re: [PATCH 1/4] kvm: x86: Dont set UMIP feature bit unconditionally

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Message-ID: <CALMp9eTb8N-WxgQ_J5_siU=8=DGNUjM=UZCN5YkAQoofZHx1hA@mail.gmail.com>
Date:   Fri, 1 Nov 2019 11:35:42 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     "Moger, Babu" <Babu.Moger@....com>
Cc:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "hpa@...or.com" <hpa@...or.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "joro@...tes.org" <joro@...tes.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "zohar@...ux.ibm.com" <zohar@...ux.ibm.com>,
        "yamada.masahiro@...ionext.com" <yamada.masahiro@...ionext.com>,
        "nayna@...ux.ibm.com" <nayna@...ux.ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH 1/4] kvm: x86: Dont set UMIP feature bit unconditionally

On Fri, Nov 1, 2019 at 10:33 AM Moger, Babu <Babu.Moger@....com> wrote:
>
> The UMIP (User-Mode Instruction Prevention) feature bit should be
> set if the emulation (kvm_x86_ops->umip_emulated) is supported
> which is done already.
>
> Remove the unconditional setting of this bit.
>
> Fixes: ae3e61e1c28338d0 ("KVM: x86: add support for UMIP")
>
> Signed-off-by: Babu Moger <babu.moger@....com>
> ---
>  arch/x86/kvm/cpuid.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index f68c0c753c38..5b81ba5ad428 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -364,7 +364,7 @@ static inline void do_cpuid_7_mask(struct kvm_cpuid_entry2 *entry, int index)
>         /* cpuid 7.0.ecx*/
>         const u32 kvm_cpuid_7_0_ecx_x86_features =
>                 F(AVX512VBMI) | F(LA57) | F(PKU) | 0 /*OSPKE*/ | F(RDPID) |
> -               F(AVX512_VPOPCNTDQ) | F(UMIP) | F(AVX512_VBMI2) | F(GFNI) |
> +               F(AVX512_VPOPCNTDQ) | F(AVX512_VBMI2) | F(GFNI) |
>                 F(VAES) | F(VPCLMULQDQ) | F(AVX512_VNNI) | F(AVX512_BITALG) |
>                 F(CLDEMOTE) | F(MOVDIRI) | F(MOVDIR64B) | 0 /*WAITPKG*/;
>

This isn't unconditional. This is masked by the features on the boot
CPU. Since UMIP can be virtualized (without emulation) on CPUs that
support UMIP, you should leave this alone.