From: Maxim Levitsky <email@example.com> To: Sean Christopherson <firstname.lastname@example.org>, Paolo Bonzini <email@example.com> Cc: Vitaly Kuznetsov <firstname.lastname@example.org>, Wanpeng Li <email@example.com>, Jim Mattson <firstname.lastname@example.org>, Joerg Roedel <email@example.com>, firstname.lastname@example.org, email@example.com, Xiaoyao Li <firstname.lastname@example.org>, Reiji Watanabe <email@example.com> Subject: Re: [PATCH 02/15] KVM: x86: Emulate RDPID only if RDTSCP is supported Date: Mon, 10 May 2021 11:08:12 +0300 [thread overview] Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <email@example.com> 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. 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. If not supported in hardware KVM only emulates RDPID as I see. 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. 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. 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 <firstname.lastname@example.org> Best regards, Maxim Levitsky
next prev parent reply other threads:[~2021-05-10 8:08 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 [this message] 2021-05-10 17:20 ` Sean Christopherson 2021-05-11 12:32 ` Maxim Levitsky 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
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --subject='Re: [PATCH 02/15] KVM: x86: Emulate RDPID only if RDTSCP is supported' \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: link
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for NNTP newsgroup(s).