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From: Sean Christopherson <seanjc@google.com>
To: Like Xu <like.xu.linux@gmail.com>
Cc: Yang Weijiang <weijiang.yang@intel.com>,
	pbonzini@redhat.com, jmattson@google.com, vkuznets@redhat.com,
	wei.w.wang@intel.com, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v8 15/15] KVM: x86/cpuid: Advise Arch LBR feature in CPUID
Date: Fri, 15 Oct 2021 14:49:39 +0000	[thread overview]
Message-ID: <YWmVA3FQoGcprtlp@google.com> (raw)
In-Reply-To: <dfe0dee9-905a-9296-4a5b-e88eb9e942a1@gmail.com>

On Fri, Oct 15, 2021, Like Xu wrote:
> On 15/10/2021 9:28 am, Yang Weijiang wrote:
> > On Fri, Oct 15, 2021 at 12:01:22AM +0000, Sean Christopherson wrote:
> > > s/Advise/Advertise
> > > 
> > > On Tue, Aug 24, 2021, Yang Weijiang wrote:
> > > > Add Arch LBR feature bit in CPU cap-mask to expose the feature.
> > > > Only max LBR depth is supported for guest, and it's consistent
> > > > with host Arch LBR settings.
> > > > 
> > > > Co-developed-by: Like Xu <like.xu@linux.intel.com>
> > > > Signed-off-by: Like Xu <like.xu@linux.intel.com>
> > > > Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> > > > ---
> > > >   arch/x86/kvm/cpuid.c | 33 ++++++++++++++++++++++++++++++++-
> > > >   1 file changed, 32 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > > index 03025eea1524..d98ebefd5d72 100644
> > > > --- a/arch/x86/kvm/cpuid.c
> > > > +++ b/arch/x86/kvm/cpuid.c
> > > > @@ -88,6 +88,16 @@ static int kvm_check_cpuid(struct kvm_cpuid_entry2 *entries, int nent)
> > > >   		if (vaddr_bits != 48 && vaddr_bits != 57 && vaddr_bits != 0)
> > > >   			return -EINVAL;
> > > >   	}
> > > > +	best = cpuid_entry2_find(entries, nent, 0x1c, 0);
> > > > +	if (best) {
> > > > +		unsigned int eax, ebx, ecx, edx;
> > > > +
> > > > +		/* Reject user-space CPUID if depth is different from host's.*/
> > > 
> > > Why disallow this?  I don't see why it would be illegal for userspace to specify
> > > fewer LBRs,
> 
> The emulation of guest LBR *depends* on the host LBR driver to save/restore
> LBRs entries (which are pass-through to the guest and store the guest branch
> instructions rips actually).
> 
> Currently, the host side does not support the use of different lbr depths on
> the same host to customize this part of the overhead. The host perf LBR
> driver assumes that the lbr depths of different tasks on different cpu's are
> the same and are the maximum value.

Yes, I assumed as much, but saving/restoring MSRs that the guest does not have
access to is not a functional issue, just a waste of cycles.

  reply	other threads:[~2021-10-15 14:49 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-24  7:56 [PATCH v8 00/15] Introduce Architectural LBR for vPMU Yang Weijiang
2021-08-24  7:56 ` [PATCH v8 01/15] perf/x86/intel: Fix the comment about guest LBR support on KVM Yang Weijiang
2021-08-24  7:56 ` [PATCH v8 02/15] perf/x86/lbr: Simplify the exposure check for the LBR_INFO registers Yang Weijiang
2021-08-24  7:56 ` [PATCH v8 03/15] KVM: x86: Add Arch LBR MSRs to msrs_to_save_all list Yang Weijiang
2021-08-24  7:56 ` [PATCH v8 04/15] KVM: vmx/pmu: Emulate MSR_ARCH_LBR_DEPTH for guest Arch LBR Yang Weijiang
2021-08-24  7:56 ` [PATCH v8 05/15] KVM: vmx/pmu: Emulate MSR_ARCH_LBR_CTL " Yang Weijiang
2021-08-25  7:59   ` kernel test robot
2021-08-27  1:06   ` kernel test robot
2021-08-24  7:56 ` [PATCH v8 06/15] KVM: x86/pmu: Refactor code to support " Yang Weijiang
2021-08-24  7:56 ` [PATCH v8 07/15] KVM: x86: Refresh CPUID on writes to MSR_IA32_XSS Yang Weijiang
2021-08-24  7:56 ` [PATCH v8 08/15] KVM: x86: Report XSS as an MSR to be saved if there are supported features Yang Weijiang
2021-08-24  7:56 ` [PATCH v8 09/15] KVM: x86: Refine the matching and clearing logic for supported_xss Yang Weijiang
2021-08-24  7:56 ` [PATCH v8 10/15] KVM: x86: Add XSAVE Support for Architectural LBR Yang Weijiang
2021-08-24  7:56 ` [PATCH v8 11/15] KVM: x86/vmx: Check Arch LBR config when return perf capabilities Yang Weijiang
2021-08-24  7:56 ` [PATCH v8 12/15] KVM: nVMX: Add necessary Arch LBR settings for nested VM Yang Weijiang
2021-08-24  7:56 ` [PATCH v8 13/15] KVM: x86/vmx: Clear Arch LBREn bit before inject #DB to guest Yang Weijiang
2021-08-24  7:56 ` [PATCH v8 14/15] KVM: x86/vmx: Flip Arch LBREn bit on guest state change Yang Weijiang
2021-08-24  7:56 ` [PATCH v8 15/15] KVM: x86/cpuid: Advise Arch LBR feature in CPUID Yang Weijiang
2021-10-15  0:01   ` Sean Christopherson
2021-10-15  1:28     ` Yang Weijiang
2021-10-15  2:05       ` Like Xu
2021-10-15 14:49         ` Sean Christopherson [this message]
2021-09-07  3:26 ` [PATCH v8 00/15] Introduce Architectural LBR for vPMU Yang Weijiang

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