From: Maxim Levitsky <firstname.lastname@example.org> To: Sean Christopherson <email@example.com> Cc: Paolo Bonzini <firstname.lastname@example.org>, Vitaly Kuznetsov <email@example.com>, Wanpeng Li <firstname.lastname@example.org>, Jim Mattson <email@example.com>, Joerg Roedel <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org, Xiaoyao Li <email@example.com>, Reiji Watanabe <firstname.lastname@example.org> 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: <email@example.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: firstname.lastname@example.org > > > Signed-off-by: Sean Christopherson <email@example.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 <firstname.lastname@example.org> > 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 <email@example.com> > > > > Best regards, > > Maxim Levitsky > > > >
next prev parent 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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