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Message-ID: <90c513d31a1b41daae1a642d2f5c72b0@EX13D32EUC003.ant.amazon.com>
Date: Thu, 4 Nov 2021 19:50:21 +0000
From: "Durrant, Paul" <pdurrant@...zon.co.uk>
To: Sean Christopherson <seanjc@...gle.com>,
Paul Durrant <paul@....org>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>
Subject: RE: [PATCH] KVM: x86: Make sure KVM_CPUID_FEATURES really are
KVM_CPUID_FEATURES
> -----Original Message-----
> From: Sean Christopherson <seanjc@...gle.com>
> Sent: 04 November 2021 19:23
> To: Paul Durrant <paul@....org>
> Cc: kvm@...r.kernel.org; linux-kernel@...r.kernel.org; Durrant, Paul <pdurrant@...zon.co.uk>; Paolo
> Bonzini <pbonzini@...hat.com>; Vitaly Kuznetsov <vkuznets@...hat.com>; Wanpeng Li
> <wanpengli@...cent.com>; Jim Mattson <jmattson@...gle.com>; Joerg Roedel <joro@...tes.org>
> Subject: RE: [EXTERNAL] [PATCH] KVM: x86: Make sure KVM_CPUID_FEATURES really are KVM_CPUID_FEATURES
>
> On Thu, Nov 04, 2021, Paul Durrant wrote:
> > From: Paul Durrant <pdurrant@...zon.com>
> >
> > Currently when kvm_update_cpuid_runtime() runs, it assumes that the
> > KVM_CPUID_FEATURES leaf is located at 0x40000001. This is not true,
> > however, if Hyper-V support is enabled. In this case the KVM leaves will
> > be offset.
> >
> > This patch introdues as new 'kvm_cpuid_base' field into struct
> > kvm_vcpu_arch to track the location of the KVM leaves and function
> > kvm_update_cpuid_base() (called from kvm_update_cpuid_runtime()) to locate
> > the leaves using the 'KVMKVMKVM\0\0\0' signature. Adjustment of
> > KVM_CPUID_FEATURES will hence now target the correct leaf.
> >
> > Signed-off-by: Paul Durrant <pdurrant@...zon.com>
> > ---
> > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > Cc: Sean Christopherson <seanjc@...gle.com>
> > Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
> > Cc: Wanpeng Li <wanpengli@...cent.com>
> > Cc: Jim Mattson <jmattson@...gle.com>
> > Cc: Joerg Roedel <joro@...tes.org>
>
> scripts/get_maintainer.pl is your friend :-)
That's what I used, but thought it prudent to trim the list to just KVM reviewers.
>
> > ---
> > arch/x86/include/asm/kvm_host.h | 1 +
> > arch/x86/kvm/cpuid.c | 50 +++++++++++++++++++++++++++++----
> > 2 files changed, 46 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 88fce6ab4bbd..21133ffa23e9 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -725,6 +725,7 @@ struct kvm_vcpu_arch {
> >
> > int cpuid_nent;
> > struct kvm_cpuid_entry2 *cpuid_entries;
> > + u32 kvm_cpuid_base;
> >
> > u64 reserved_gpa_bits;
> > int maxphyaddr;
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index 2d70edb0f323..2cfb8ec4f570 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -99,11 +99,46 @@ static int kvm_check_cpuid(struct kvm_cpuid_entry2 *entries, int nent)
> > return 0;
> > }
> >
> > +static void kvm_update_cpuid_base(struct kvm_vcpu *vcpu)
> > +{
> > + u32 function;
> > +
> > + for (function = 0x40000000; function < 0x40010000; function += 0x100) {
>
> No small part of me wants to turn hypervisor_cpuid_base() into a macro, but that's
> probably more pain than gain. But I do think it would be worth providing a macro
> to iterate over possible bases and share that with the guest-side code.
>
Ok.
> > + struct kvm_cpuid_entry2 *best = kvm_find_cpuid_entry(vcpu, function, 0);
>
> Declare "struct kvm_cpuid_entry2 *best" outside of the loop to shorten this line.
> I'd also vote to rename "best" to "entry". KVM's "best" terminology is a remnant
> of misguided logic that applied Intel's bizarre out-of-range behavior to internal
> KVM lookups.
>
Sure.
> > +
> > + if (best) {
> > + char signature[12];
> > +
> > + *(u32 *)&signature[0] = best->ebx;
>
> Just make signature a u32[3], then the casting craziness goes away.
>
True.
> > + *(u32 *)&signature[4] = best->ecx;
> > + *(u32 *)&signature[8] = best->edx;
> > +
> > + if (!memcmp(signature, "KVMKVMKVM\0\0\0", 12))
>
> The "KVMKVMKVM\0\0\0" magic string belongs in a #define that's shared with the
> guest-side code. I
>
It's in a few places, so yes a #define is a good idea.
> > + break;
> > + }
> > + }
> > + vcpu->arch.kvm_cpuid_base = function;
>
> Unconditionally setting kvm_cpuid_base is silly because then kvm_get_cpuid_base()
> needs to check multiple "error" values.
>
> E.g. all of the above can be done as:
>
> struct kvm_cpuid_entry2 *entry;
> u32 base, signature[3];
>
> vcpu->arch.kvm_cpuid_base = 0;
>
> virt_for_each_possible_hypervisor_base(base) {
> entry = kvm_find_cpuid_entry(vcpu, base, 0);
> if (!entry)
> continue;
>
> signature[0] = entry->ebx;
> signature[1] = entry->ecx;
> signature[2] = entry->edx;
>
> if (!memcmp(signature, KVM_CPUID_SIG, sizeof(signature))) {
> vcpu->arch.kvm_cpuid_base = base;
> break;
> }
> }
>
> > +}
> > +
> > +static inline bool kvm_get_cpuid_base(struct kvm_vcpu *vcpu, u32 *function)
> > +{
> > + if (vcpu->arch.kvm_cpuid_base < 0x40000000 ||
> > + vcpu->arch.kvm_cpuid_base >= 0x40010000)
> > + return false;
> > +
> > + *function = vcpu->arch.kvm_cpuid_base;
> > + return true;
>
> If '0' is the "doesn't exist" value, then this helper goes away.
>
> > +}
> > +
> > void kvm_update_pv_runtime(struct kvm_vcpu *vcpu)
> > {
> > + u32 base;
> > struct kvm_cpuid_entry2 *best;
> >
> > - best = kvm_find_cpuid_entry(vcpu, KVM_CPUID_FEATURES, 0);
> > + if (!kvm_get_cpuid_base(vcpu, &base))
> > + return;
>
> ... and then this becomes:
>
> if (!vcpu->arch.kvm_cpuid_base)
> return;
>
Ok.
> Actually, since this is a repated pattern and is likely going to be limited to
> getting KVM_CPUID_FEATURES, just add:
>
> struct kvm_find_cpuid_entry kvm_find_kvm_cpuid_features(void)
> {
> u32 base = vcpu->arch.kvm_cpuid_base;
>
> if (!base)
> return NULL;
>
> return kvm_find_cpuid_entry(vcpu, base | KVM_CPUID_FEATURES, 0);
> }
>
> and then all of the indentation churn goes away.
>
Yeah, probably neater.
> > +
> > + best = kvm_find_cpuid_entry(vcpu, base + KVM_CPUID_FEATURES, 0);
> >
> > /*
> > * save the feature bitmap to avoid cpuid lookup for every PV
> > @@ -116,6 +151,7 @@ void kvm_update_pv_runtime(struct kvm_vcpu *vcpu)
> > void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
> > {
> > struct kvm_cpuid_entry2 *best;
> > + u32 base;
> >
> > best = kvm_find_cpuid_entry(vcpu, 1, 0);
> > if (best) {
> > @@ -142,10 +178,14 @@ void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
> > cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
> > best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
> >
> > - best = kvm_find_cpuid_entry(vcpu, KVM_CPUID_FEATURES, 0);
> > - if (kvm_hlt_in_guest(vcpu->kvm) && best &&
> > - (best->eax & (1 << KVM_FEATURE_PV_UNHALT)))
> > - best->eax &= ~(1 << KVM_FEATURE_PV_UNHALT);
> > + kvm_update_cpuid_base(vcpu);
>
> The KVM base doesn't need to be rechecked for runtime updates. Runtime updates
> are to handle changes in guest state, e.g. reported XSAVE size in response to a
> CR4.OSXSAVE change. The raw CPUID entries themselves cannot change at runtime.
> I suspect you did this here because kvm_update_cpuid_runtime() is called before
> kvm_vcpu_after_set_cpuid(), but that has the very bad side effect of doing an
> _expensive_ lookup on every runtime update, which can get very painful if there's
> no KVM_CPUID_FEATURES to be found.
>
> If you include the prep patch (pasted at the bottom), then this can simply be
> (note the somewhat silly name; I think it's worth clarifying that it's the
> KVM_CPUID_* base that's being updated):
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0c99d2731076..5dd8c26e9f86 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -245,6 +245,7 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
> vcpu->arch.cpuid_entries = e2;
> vcpu->arch.cpuid_nent = nent;
>
> + kvm_update_kvm_cpuid_base(vcpu);
> kvm_update_cpuid_runtime(vcpu);
> kvm_vcpu_after_set_cpuid(vcpu);
>
> > +
> > + if (kvm_get_cpuid_base(vcpu, &base)) {
> > + best = kvm_find_cpuid_entry(vcpu, base + KVM_CPUID_FEATURES, 0);
>
> This is wrong. base will be >0x40000000 and <0x40010000, and KVM_CPUID_FEATURES
> is 0x40000001, i.e. this will lookup 0x80000001 for the default base. The '+'
> needs to be an '|'.
>
Yes, it does.
> > + if (kvm_hlt_in_guest(vcpu->kvm) && best &&
> > + (best->eax & (1 << KVM_FEATURE_PV_UNHALT)))
> > + best->eax &= ~(1 << KVM_FEATURE_PV_UNHALT);
> > + }
> >
> > if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT)) {
> > best = kvm_find_cpuid_entry(vcpu, 0x1, 0);
> > --
> > 2.20.1
>
Thanks,
Paul
>
> From 02d58c124f5aab1b0ef28cfc8a6ff6b6c58df969 Mon Sep 17 00:00:00 2001
> From: Sean Christopherson <seanjc@...gle.com>
> Date: Thu, 4 Nov 2021 12:17:23 -0700
> Subject: [PATCH] KVM: x86: Add helper to consolidate core logic of
> SET_CPUID{2} flows
>
> Move the core logic of SET_CPUID and SET_CPUID2 to a common helper, the
> only difference between the two ioctls() is the format of the userspace
> struct. A future fix will add yet more code to the core logic.
>
> No functional change intended.
>
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
> arch/x86/kvm/cpuid.c | 47 ++++++++++++++++++++++----------------------
> 1 file changed, 24 insertions(+), 23 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 751aa85a3001..0c99d2731076 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -232,6 +232,25 @@ u64 kvm_vcpu_reserved_gpa_bits_raw(struct kvm_vcpu *vcpu)
> return rsvd_bits(cpuid_maxphyaddr(vcpu), 63);
> }
>
> +static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
> + int nent)
> +{
> + int r;
> +
> + r = kvm_check_cpuid(e2, nent);
> + if (r)
> + return r;
> +
> + kvfree(vcpu->arch.cpuid_entries);
> + vcpu->arch.cpuid_entries = e2;
> + vcpu->arch.cpuid_nent = nent;
> +
> + kvm_update_cpuid_runtime(vcpu);
> + kvm_vcpu_after_set_cpuid(vcpu);
> +
> + return 0;
> +}
> +
> /* when an old userspace process fills a new kernel module */
> int kvm_vcpu_ioctl_set_cpuid(struct kvm_vcpu *vcpu,
> struct kvm_cpuid *cpuid,
> @@ -268,18 +287,9 @@ int kvm_vcpu_ioctl_set_cpuid(struct kvm_vcpu *vcpu,
> e2[i].padding[2] = 0;
> }
>
> - r = kvm_check_cpuid(e2, cpuid->nent);
> - if (r) {
> + r = kvm_set_cpuid(vcpu, e2, cpuid->nent);
> + if (r)
> kvfree(e2);
> - goto out_free_cpuid;
> - }
> -
> - kvfree(vcpu->arch.cpuid_entries);
> - vcpu->arch.cpuid_entries = e2;
> - vcpu->arch.cpuid_nent = cpuid->nent;
> -
> - kvm_update_cpuid_runtime(vcpu);
> - kvm_vcpu_after_set_cpuid(vcpu);
>
> out_free_cpuid:
> kvfree(e);
> @@ -303,20 +313,11 @@ int kvm_vcpu_ioctl_set_cpuid2(struct kvm_vcpu *vcpu,
> return PTR_ERR(e2);
> }
>
> - r = kvm_check_cpuid(e2, cpuid->nent);
> - if (r) {
> + r = kvm_set_cpuid(vcpu, e2, cpuid->nent);
> + if (r)
> kvfree(e2);
> - return r;
> - }
>
> - kvfree(vcpu->arch.cpuid_entries);
> - vcpu->arch.cpuid_entries = e2;
> - vcpu->arch.cpuid_nent = cpuid->nent;
> -
> - kvm_update_cpuid_runtime(vcpu);
> - kvm_vcpu_after_set_cpuid(vcpu);
> -
> - return 0;
> + return r;
> }
>
> int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
> --
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