From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: "Paolo Bonzini" <pbonzini@redhat.com>,
"Radim Krčmář" <rkrcmar@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,
"Reto Buerki" <reet@codelabs.ch>,
"Liran Alon" <liran.alon@oracle.com>
Subject: Re: [PATCH v2 4/8] KVM: VMX: Optimize vmx_set_rflags() for unrestricted guest
Date: Mon, 30 Sep 2019 17:55:41 +0200 [thread overview]
Message-ID: <87k19p3hj6.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20190930151945.GB14693@linux.intel.com>
Sean Christopherson <sean.j.christopherson@intel.com> writes:
> On Mon, Sep 30, 2019 at 10:57:17AM +0200, Vitaly Kuznetsov wrote:
>> Sean Christopherson <sean.j.christopherson@intel.com> writes:
>>
>> > Rework vmx_set_rflags() to avoid the extra code need to handle emulation
>> > of real mode and invalid state when unrestricted guest is disabled. The
>> > primary reason for doing so is to avoid the call to vmx_get_rflags(),
>> > which will incur a VMREAD when RFLAGS is not already available. When
>> > running nested VMs, the majority of calls to vmx_set_rflags() will occur
>> > without an associated vmx_get_rflags(), i.e. when stuffing GUEST_RFLAGS
>> > during transitions between vmcs01 and vmcs02.
>> >
>> > Note, vmx_get_rflags() guarantees RFLAGS is marked available.
>> >
>> > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
>> > ---
>> > arch/x86/kvm/vmx/vmx.c | 28 ++++++++++++++++++----------
>> > 1 file changed, 18 insertions(+), 10 deletions(-)
>> >
>> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> > index 83fe8b02b732..814d3e6d0264 100644
>> > --- a/arch/x86/kvm/vmx/vmx.c
>> > +++ b/arch/x86/kvm/vmx/vmx.c
>> > @@ -1426,18 +1426,26 @@ unsigned long vmx_get_rflags(struct kvm_vcpu *vcpu)
>> > void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
>> > {
>> > struct vcpu_vmx *vmx = to_vmx(vcpu);
>> > - unsigned long old_rflags = vmx_get_rflags(vcpu);
>> > + unsigned long old_rflags;
>> >
>> > - __set_bit(VCPU_EXREG_RFLAGS, (ulong *)&vcpu->arch.regs_avail);
>> > - vmx->rflags = rflags;
>> > - if (vmx->rmode.vm86_active) {
>> > - vmx->rmode.save_rflags = rflags;
>> > - rflags |= X86_EFLAGS_IOPL | X86_EFLAGS_VM;
>> > + if (enable_unrestricted_guest) {
>> > + __set_bit(VCPU_EXREG_RFLAGS, (ulong *)&vcpu->arch.regs_avail);
>> > +
>> > + vmx->rflags = rflags;
>> > + vmcs_writel(GUEST_RFLAGS, rflags);
>> > + } else {
>> > + old_rflags = vmx_get_rflags(vcpu);
>> > +
>> > + vmx->rflags = rflags;
>> > + if (vmx->rmode.vm86_active) {
>> > + vmx->rmode.save_rflags = rflags;
>> > + rflags |= X86_EFLAGS_IOPL | X86_EFLAGS_VM;
>> > + }
>> > + vmcs_writel(GUEST_RFLAGS, rflags);
>> > +
>> > + if ((old_rflags ^ vmx->rflags) & X86_EFLAGS_VM)
>> > + vmx->emulation_required = emulation_required(vcpu);
>> > }
>> > - vmcs_writel(GUEST_RFLAGS, rflags);
>>
>> We're doing vmcs_writel() in both branches so it could've stayed here, right?
>
> Yes, but the resulting code is a bit ugly. emulation_required() consumes
> vmcs.GUEST_RFLAGS, i.e. the if statement that reads old_rflags would also
> need to be outside of the else{} case.
>
> This isn't too bad:
>
> if (!enable_unrestricted_guest &&
> ((old_rflags ^ vmx->rflags) & X86_EFLAGS_VM))
> vmx->emulation_required = emulation_required(vcpu);
>
> but gcc isn't smart enough to understand old_rflags won't be used if
> enable_unrestricted_guest, so old_rflags either needs to be tagged with
> uninitialized_var() or explicitly initialized in the if(){} case.
>
> Duplicating a small amount of code felt like the lesser of two evils.
>
I see, thanks for these additional details!
--
Vitaly
next prev parent reply other threads:[~2019-09-30 15:55 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-27 21:45 [PATCH v2 0/8] KVM: x86: nVMX GUEST_CR3 bug fix, and then some Sean Christopherson
2019-09-27 21:45 ` [PATCH v2 1/8] KVM: nVMX: Always write vmcs02.GUEST_CR3 during nested VM-Enter Sean Christopherson
2019-09-27 23:37 ` Jim Mattson
2019-09-27 21:45 ` [PATCH v2 2/8] KVM: VMX: Skip GUEST_CR3 VMREAD+VMWRITE if the VMCS is up-to-date Sean Christopherson
2019-09-27 21:45 ` [PATCH v2 3/8] KVM: VMX: Consolidate to_vmx() usage in RFLAGS accessors Sean Christopherson
2019-09-30 8:48 ` Vitaly Kuznetsov
2019-09-27 21:45 ` [PATCH v2 4/8] KVM: VMX: Optimize vmx_set_rflags() for unrestricted guest Sean Christopherson
2019-09-30 8:57 ` Vitaly Kuznetsov
2019-09-30 15:19 ` Sean Christopherson
2019-09-30 15:55 ` Vitaly Kuznetsov [this message]
2019-10-09 10:40 ` Paolo Bonzini
2019-10-09 16:38 ` Sean Christopherson
2019-10-09 20:59 ` Paolo Bonzini
2019-10-09 21:30 ` Sean Christopherson
2019-09-27 21:45 ` [PATCH v2 5/8] KVM: x86: Add WARNs to detect out-of-bounds register indices Sean Christopherson
2019-09-30 9:19 ` Vitaly Kuznetsov
2019-10-09 10:50 ` Paolo Bonzini
2019-10-09 16:36 ` Sean Christopherson
2019-09-27 21:45 ` [PATCH v2 6/8] KVM: x86: Fold 'enum kvm_ex_reg' definitions into 'enum kvm_reg' Sean Christopherson
2019-09-30 9:25 ` Vitaly Kuznetsov
2019-10-09 10:52 ` Paolo Bonzini
2019-10-09 11:27 ` Vitaly Kuznetsov
2019-09-27 21:45 ` [PATCH v2 7/8] KVM: x86: Add helpers to test/mark reg availability and dirtiness Sean Christopherson
2019-09-30 9:32 ` Vitaly Kuznetsov
2019-10-09 11:00 ` Paolo Bonzini
2019-09-27 21:45 ` [PATCH v2 8/8] KVM: x86: Fold decache_cr3() into cache_reg() Sean Christopherson
2019-09-30 10:58 ` Vitaly Kuznetsov
2019-09-30 15:04 ` Sean Christopherson
2019-09-30 15:27 ` Vitaly Kuznetsov
2019-09-30 15:33 ` Sean Christopherson
2019-10-09 11:03 ` Paolo Bonzini
2019-09-30 10:42 ` [PATCH v2 0/8] KVM: x86: nVMX GUEST_CR3 bug fix, and then some Reto Buerki
2019-10-29 15:03 ` Martin Lucina
2019-10-30 9:09 ` Sean Christopherson
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