linux-kernel - RE: [PATCH v2 23/23] kvm: x86: Disable RDMSR interception of IA32_XFD

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Date:   Tue, 21 Dec 2021 06:29:06 +0000
From:   "Liu, Jing2" <jing2.liu@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
CC:     "Christopherson,, Sean" <seanjc@...gle.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "jing2.liu@...ux.intel.com" <jing2.liu@...ux.intel.com>,
        "Zeng, Guang" <guang.zeng@...el.com>,
        "Wang, Wei W" <wei.w.wang@...el.com>,
        "Zhong, Yang" <yang.zhong@...el.com>
Subject: RE: [PATCH v2 23/23] kvm: x86: Disable RDMSR interception of
 IA32_XFD_ERR

Hi Paolo,
On 12/20/2021 5:08 PM, Paolo Bonzini wrote:
> 
> On 12/17/21 16:30, Jing Liu wrote:
> > Also disable read emulation of IA32_XFD_ERR MSR at the same point
> > where r/w emulation of IA32_XFD MSR is disabled. This saves one
> > unnecessary VM-exit in guest #NM handler, given that the MSR is
> > already restored with the guest value before the guest is resumed.
> >
> > Signed-off-by: Jing Liu <jing2.liu@...el.com>
> 
> Why not do this unconditionally (i.e. in patch 13, with the call to
> vmx_disable_intercept_for_msr in function vmx_create_vcpu)?
> 
Thanks for reviewing the patches. 

If disable unconditionally in vmx_create_vcpu, it means even guest has
no cpuid, the msr read is passthrough to guest and it can read a value, which 
seems strange, though spec doesn't mention the read behaviour w/o cpuid.

How about disabling read interception at vmx_vcpu_after_set_cpuid?

if (boot_cpu_has(X86_FEATURE_XFD) && guest_cpuid_has(vcpu, X86_FEATURE_XFD))
        vmx_set_intercept_for_msr(vcpu, MSR_IA32_XFD_ERR, MSR_TYPE_R, false);

Thanks,
Jing

> Thanks,
> 
> Paolo
> 
> > ---
> >   arch/x86/kvm/vmx/vmx.c | 2 ++
> >   arch/x86/kvm/vmx/vmx.h | 2 +-
> >   2 files changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index
> > 97a823a3f23f..b66a005f076b 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -163,6 +163,7 @@ static u32
> vmx_possible_passthrough_msrs[MAX_POSSIBLE_PASSTHROUGH_MSRS] = {
> >   	MSR_GS_BASE,
> >   	MSR_KERNEL_GS_BASE,
> >   	MSR_IA32_XFD,
> > +	MSR_IA32_XFD_ERR,
> >   #endif
> >   	MSR_IA32_SYSENTER_CS,
> >   	MSR_IA32_SYSENTER_ESP,
> > @@ -1934,6 +1935,7 @@ static u64 vcpu_supported_debugctl(struct
> kvm_vcpu *vcpu)
> >   static void vmx_set_xfd_passthrough(struct kvm_vcpu *vcpu)
> >   {
> >   	vmx_disable_intercept_for_msr(vcpu, MSR_IA32_XFD,
> MSR_TYPE_RW);
> > +	vmx_disable_intercept_for_msr(vcpu, MSR_IA32_XFD_ERR,
> MSR_TYPE_R);
> >   	vcpu->arch.xfd_out_of_sync = true;
> >   }
> >   #endif
> > diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h index
> > bf9d3051cd6c..0a00242a91e7 100644
> > --- a/arch/x86/kvm/vmx/vmx.h
> > +++ b/arch/x86/kvm/vmx/vmx.h
> > @@ -340,7 +340,7 @@ struct vcpu_vmx {
> >   	struct lbr_desc lbr_desc;
> >
> >   	/* Save desired MSR intercept (read: pass-through) state */
> > -#define MAX_POSSIBLE_PASSTHROUGH_MSRS	14
> > +#define MAX_POSSIBLE_PASSTHROUGH_MSRS	15
> >   	struct {
> >   		DECLARE_BITMAP(read,
> MAX_POSSIBLE_PASSTHROUGH_MSRS);
> >   		DECLARE_BITMAP(write,
> MAX_POSSIBLE_PASSTHROUGH_MSRS);
> >