linux-kernel - Re: [PATCH] KVM: vmx: Fix the broken usage of vmx_xsaves

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Date:   Thu, 20 Jun 2019 16:27:26 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Xiaoyao Li <xiaoyao.li@...ux.intel.com>,
        Tao Xu <tao3.xu@...el.com>, Radim Krcmar <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: vmx: Fix the broken usage of vmx_xsaves_supported

On Thu, 20 Jun 2019 at 16:17, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 20/06/19 08:46, Xiaoyao Li wrote:
> >>
> >> It depends on whether or not processors support the 1-setting instead
> >> of “enable XSAVES/XRSTORS” is 1 in VM-exection control field. Anyway,
> >
> > Yes, whether this field exist or not depends on whether processors
> > support the 1-setting.
> >
> > But if "enable XSAVES/XRSTORS" is clear to 0, XSS_EXIT_BITMAP doesn't
> > work. I think in this case, there is no need to set this vmcs field?
>
> vmx->secondary_exec_control can change; you are making the code more
> complex by relying on the value of the field at the point of vmx_vcpu_setup.
>
> I do _think_ your version is incorrect, because at this point CPUID has
> not been initialized yet and therefore
> vmx_compute_secondary_exec_control has not set SECONDARY_EXEC_XSAVES.
> However I may be wrong because I didn't review the code very closely:
> the old code is obvious and so there is no point in changing it.

Agreed, in addition, guest can enable/disable cpuid bits by grub
parameter, should we call kvm_x86_ops->cpuid_update() in
kvm_vcpu_reset() path to reflect the new guest cpuid influence to
exec_control? e.g. the first boot guest disable xsaves in grub, kvm
disables xsaves in exec_control; then guest reboot w/ xsaves enabled,
it still get an #UD when executing.

Regards,
Wanpeng Li