linux-kernel - Re: [PATCH 0/2] KVM: x86: nSVM: fixes for SYSENTER emulation

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Date:   Thu, 01 Apr 2021 14:20:28 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 0/2] KVM: x86: nSVM: fixes for SYSENTER emulation

On Thu, 2021-04-01 at 14:16 +0300, Maxim Levitsky wrote:
> This is a result of a deep rabbit hole dive in regard to why
> currently the nested migration of 32 bit guests
> is totally broken on AMD.

Please ignore this patch series, I didn't update the patch version.

Best regards,
	Maxim Levitsky

> 
> It turns out that due to slight differences between the original AMD64
> implementation and the Intel's remake, SYSENTER instruction behaves a
> bit differently on Intel, and to support migration from Intel to AMD we
> try to emulate those differences away.
> 
> Sadly that collides with virtual vmload/vmsave feature that is used in nesting.
> The problem was that when it is enabled,
> on migration (and otherwise when userspace reads MSR_IA32_SYSENTER_{EIP|ESP},
> wrong value were returned, which leads to #DF in the
> nested guest when the wrong value is loaded back.
> 
> The patch I prepared carefully fixes this, by mostly disabling that
> SYSCALL emulation when we don't spoof the Intel's vendor ID, and if we do,
> and yet somehow SVM is enabled (this is a very rare edge case), then
> virtual vmload/save is force disabled.
> 
> V2: incorporated review feedback from Paulo.
> 
> Best regards,
>         Maxim Levitsky
> 
> Maxim Levitsky (2):
>   KVM: x86: add guest_cpuid_is_intel
>   KVM: nSVM: improve SYSENTER emulation on AMD
> 
>  arch/x86/kvm/cpuid.h   |  8 ++++
>  arch/x86/kvm/svm/svm.c | 99 +++++++++++++++++++++++++++---------------
>  arch/x86/kvm/svm/svm.h |  6 +--
>  3 files changed, 76 insertions(+), 37 deletions(-)
> 
> -- 
> 2.26.2
>