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From: Maxim Levitsky <mlevitsk@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Xiaoyao Li <xiaoyao.li@intel.com>,
	Reiji Watanabe <reijiw@google.com>
Subject: Re: [PATCH 02/15] KVM: x86: Emulate RDPID only if RDTSCP is supported
Date: Tue, 11 May 2021 15:32:26 +0300	[thread overview]
Message-ID: <4f233c7120067c331a767e55ae8513c945e0f1ba.camel@redhat.com> (raw)
In-Reply-To: <YJlrZJrVhUKCbnba@google.com>

On Mon, 2021-05-10 at 17:20 +0000, Sean Christopherson wrote:
> On Mon, May 10, 2021, Maxim Levitsky wrote:
> > On Tue, 2021-05-04 at 10:17 -0700, Sean Christopherson wrote:
> > > Do not advertise emulation support for RDPID if RDTSCP is unsupported.
> > > RDPID emulation subtly relies on MSR_TSC_AUX to exist in hardware, as
> > > both vmx_get_msr() and svm_get_msr() will return an error if the MSR is
> > > unsupported, i.e. ctxt->ops->get_msr() will fail and the emulator will
> > > inject a #UD.
> > > 
> > > Note, RDPID emulation also relies on RDTSCP being enabled in the guest,
> > > but this is a KVM bug and will eventually be fixed.
> > > 
> > > Fixes: fb6d4d340e05 ("KVM: x86: emulate RDPID")
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > > ---
> > >  arch/x86/kvm/cpuid.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > index f765bf7a529c..c96f79c9fff2 100644
> > > --- a/arch/x86/kvm/cpuid.c
> > > +++ b/arch/x86/kvm/cpuid.c
> > > @@ -637,7 +637,8 @@ static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
> > >  	case 7:
> > >  		entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
> > >  		entry->eax = 0;
> > > -		entry->ecx = F(RDPID);
> > > +		if (kvm_cpu_cap_has(X86_FEATURE_RDTSCP))
> > > +			entry->ecx = F(RDPID);
> > >  		++array->nent;
> > >  	default:
> > >  		break;
> > 
> > Just to make sure that I understand this correctly:
> > 
> > This is what I know:
> > 
> > Both RDTSCP and RDPID are instructions that read IA32_TSC_AUX
> > (and RDTSCP also reads the TSC).
> > 
> > Both instructions have their own CPUID bits (X86_FEATURE_RDPID, X86_FEATURE_RDTSCP)
> > If either of these CPUID bits are present, IA32_TSC_AUX should be supported.
> 
> Yep.
> 
> > RDPID is a newer feature, thus I can at least for the sanity sake assume that
> > usually a CPU will either have neither of the features, have only RDTSCP,
> > and IA32_AUX, or have both RDSCP and RDPID.
> 
> Yep.
> 
> > If not supported in hardware KVM only emulates RDPID as I see.
> 
> Yep.
> 
> > Why btw? Performance wise guest that only wants the IA32_AUX in userspace,
> > is better to use RDTSCP and pay the penalty of saving/restoring of the
> > unwanted registers, than use RDPID with a vmexit.
> 
> FWIW, Linux doesn't even fall back to RDTSCP.  If RDPID isn't supported, Linux
> throws the info into the limit of a dummy segment in the GDT and uses LSL to get
> at the data.  Turns out that RDTSCP is too slow for its intended use case :-)
> 
> > My own guess for an answer to this question is that RDPID emulation is there
> > to aid migration from a host that does support RDPID to a host that doesn't.
> 
> That's my assumption as well.  Paolo's commit is a bit light on why emulation
> was added in the first place, but emulating to allow migrating to old hardware
> is the only motivation I can come up with.

Cool thanks!
Best regards,
	Maxim Levitsky

> 
> commit fb6d4d340e0532032c808a9933eaaa7b8de435ab
> Author: Paolo Bonzini <pbonzini@redhat.com>
> Date:   Tue Jul 12 11:04:26 2016 +0200
> 
>     KVM: x86: emulate RDPID
> 
>     This is encoded as F3 0F C7 /7 with a register argument.  The register
>     argument is the second array in the group9 GroupDual, while F3 is the
>     fourth element of a Prefix.
> 
> > Having said all that, assuming that we don't want to emulate the RDTSCP too,
> > when it is not supported, then this patch does make sense.
> > 
> > Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
> > 
> > Best regards,
> > 	Maxim Levitsky
> > 
> > 



  reply	other threads:[~2021-05-11 12:32 UTC|newest]

Thread overview: 68+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-04 17:17 [PATCH 00/15] KVM: x86: RDPID/RDTSCP fixes and uret MSR cleanups Sean Christopherson
2021-05-04 17:17 ` [PATCH 01/15] KVM: VMX: Do not adverise RDPID if ENABLE_RDTSCP control is unsupported Sean Christopherson
2021-05-04 17:37   ` Jim Mattson
2021-05-04 17:53     ` Jim Mattson
2021-05-04 18:14       ` Sean Christopherson
2021-05-05  3:04   ` Reiji Watanabe
2021-05-10  8:03   ` Maxim Levitsky
2021-05-04 17:17 ` [PATCH 02/15] KVM: x86: Emulate RDPID only if RDTSCP is supported Sean Christopherson
2021-05-04 17:50   ` Jim Mattson
2021-05-05  3:51   ` Reiji Watanabe
2021-05-05  8:01     ` Paolo Bonzini
2021-05-10  8:08   ` Maxim Levitsky
2021-05-10 17:20     ` Sean Christopherson
2021-05-11 12:32       ` Maxim Levitsky [this message]
2021-05-04 17:17 ` [PATCH 03/15] KVM: SVM: Inject #UD on RDTSCP when it should be disabled in the guest Sean Christopherson
2021-05-04 21:45   ` Jim Mattson
2021-05-04 21:53     ` Sean Christopherson
2021-05-04 21:56       ` Jim Mattson
2021-05-04 22:10         ` Sean Christopherson
2021-05-04 22:24           ` Jim Mattson
2021-05-04 21:57       ` Paolo Bonzini
2021-05-04 21:58         ` Jim Mattson
2021-05-10  8:08           ` Maxim Levitsky
2021-05-10 16:56             ` Sean Christopherson
2021-05-11 12:34               ` Maxim Levitsky
2021-05-18 10:59               ` Paolo Bonzini
2021-05-18 19:24                 ` Sean Christopherson
2021-05-05  4:26   ` Reiji Watanabe
2021-05-10  8:08   ` Maxim Levitsky
2021-05-04 17:17 ` [PATCH 04/15] KVM: x86: Move RDPID emulation intercept to its own enum Sean Christopherson
2021-05-04 23:24   ` Jim Mattson
2021-05-10  8:14   ` Maxim Levitsky
2021-05-04 17:17 ` [PATCH 05/15] KVM: VMX: Disable preemption when probing user return MSRs Sean Christopherson
2021-05-04 23:36   ` Jim Mattson
2021-05-10  8:18   ` Maxim Levitsky
2021-05-04 17:17 ` [PATCH 06/15] KVM: SVM: Probe and load MSR_TSC_AUX regardless of RDTSCP support in host Sean Christopherson
2021-05-10  8:20   ` Maxim Levitsky
2021-05-04 17:17 ` [PATCH 07/15] KVM: x86: Add support for RDPID without RDTSCP Sean Christopherson
2021-05-10  8:20   ` Maxim Levitsky
2021-05-04 17:17 ` [PATCH 08/15] KVM: VMX: Configure list of user return MSRs at module init Sean Christopherson
2021-05-10  8:23   ` Maxim Levitsky
2021-05-10 15:13     ` Sean Christopherson
2021-05-11 12:34       ` Maxim Levitsky
2021-05-11 20:10         ` Sean Christopherson
2021-05-04 17:17 ` [PATCH 09/15] KVM: VMX: Use flag to indicate "active" uret MSRs instead of sorting list Sean Christopherson
2021-05-08  3:31   ` Reiji Watanabe
2021-05-10 16:43     ` Sean Christopherson
2021-05-10 17:55       ` Reiji Watanabe
2021-05-10  8:25   ` Maxim Levitsky
2021-05-04 17:17 ` [PATCH 10/15] KVM: VMX: Use common x86's uret MSR list as the one true list Sean Christopherson
2021-05-10  8:25   ` Maxim Levitsky
2021-05-04 17:17 ` [PATCH 11/15] KVM: VMX: Disable loading of TSX_CTRL MSR the more conventional way Sean Christopherson
2021-05-05  8:49   ` Paolo Bonzini
2021-05-05 15:36     ` Sean Christopherson
2021-05-05 15:50       ` Paolo Bonzini
2021-05-10  8:26   ` Maxim Levitsky
2021-05-04 17:17 ` [PATCH 12/15] KVM: x86: Export the number of uret MSRs to vendor modules Sean Christopherson
2021-05-10  8:27   ` Maxim Levitsky
2021-05-04 17:17 ` [PATCH 13/15] KVM: x86: Move uret MSR slot management to common x86 Sean Christopherson
2021-05-10  8:28   ` Maxim Levitsky
2021-05-04 17:17 ` [PATCH 14/15] KVM: x86: Tie Intel and AMD behavior for MSR_TSC_AUX to guest CPU model Sean Christopherson
2021-05-10  8:29   ` Maxim Levitsky
2021-05-10 16:50     ` Sean Christopherson
2021-05-10 17:11       ` Jim Mattson
2021-05-11 12:34         ` Maxim Levitsky
2021-05-04 17:17 ` [PATCH 15/15] KVM: x86: Hide RDTSCP and RDPID if MSR_TSC_AUX probing failed Sean Christopherson
2021-05-10  8:29   ` Maxim Levitsky
2021-05-05  8:51 ` [PATCH 00/15] KVM: x86: RDPID/RDTSCP fixes and uret MSR cleanups Paolo Bonzini

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