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Date: Tue, 06 Apr 2021 13:22:58 +0300
From: Maxim Levitsky <mlevitsk@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
Jim Mattson <jmattson@...gle.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Joerg Roedel <joro@...tes.org>,
"H. Peter Anvin" <hpa@...or.com>,
"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<linux-kernel@...r.kernel.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Jonathan Corbet <corbet@....net>,
Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH 1/6] KVM: nVMX: delay loading of PDPTRs to
KVM_REQ_GET_NESTED_STATE_PAGES
On Fri, 2021-04-02 at 17:27 +0000, Sean Christopherson wrote:
> On Thu, Apr 01, 2021, Maxim Levitsky wrote:
> > Similar to the rest of guest page accesses after migration,
> > this should be delayed to KVM_REQ_GET_NESTED_STATE_PAGES
> > request.
>
> FWIW, I still object to this approach, and this patch has a plethora of issues.
>
> I'm not against deferring various state loading to KVM_RUN, but wholesale moving
> all of GUEST_CR3 processing without in-depth consideration of all the side
> effects is a really bad idea.
It could be, I won't argue about this.
>
> > Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> > ---
> > arch/x86/kvm/vmx/nested.c | 14 +++++++++-----
> > 1 file changed, 9 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> > index fd334e4aa6db..b44f1f6b68db 100644
> > --- a/arch/x86/kvm/vmx/nested.c
> > +++ b/arch/x86/kvm/vmx/nested.c
> > @@ -2564,11 +2564,6 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
> > return -EINVAL;
> > }
> >
> > - /* Shadow page tables on either EPT or shadow page tables. */
> > - if (nested_vmx_load_cr3(vcpu, vmcs12->guest_cr3, nested_cpu_has_ept(vmcs12),
> > - entry_failure_code))
> > - return -EINVAL;
> > -
> > /*
> > * Immediately write vmcs02.GUEST_CR3. It will be propagated to vmcs12
> > * on nested VM-Exit, which can occur without actually running L2 and
> > @@ -3109,11 +3104,16 @@ static bool nested_get_evmcs_page(struct kvm_vcpu *vcpu)
> > static bool nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
> > {
> > struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
> > + enum vm_entry_failure_code entry_failure_code;
> > struct vcpu_vmx *vmx = to_vmx(vcpu);
> > struct kvm_host_map *map;
> > struct page *page;
> > u64 hpa;
> >
> > + if (nested_vmx_load_cr3(vcpu, vmcs12->guest_cr3, nested_cpu_has_ept(vmcs12),
> > + &entry_failure_code))
>
> This results in KVM_RUN returning 0 without filling vcpu->run->exit_reason.
> Speaking from experience, debugging those types of issues is beyond painful.
>
> It also means CR3 is double loaded in the from_vmentry case.
>
> And it will cause KVM to incorrectly return NVMX_VMENTRY_KVM_INTERNAL_ERROR
> if a consistency check fails when nested_get_vmcs12_pages() is called on
> from_vmentry. E.g. run unit tests with this and it will silently disappear.
I do remember now that you said something about this, but I wasn't able
to find it in my email. Sorry about this.
I agree with you.
I think that a question I should ask is why do we really need to
delay accessing guest memory after a migration.
So far I mostly just assumed that we need to do so, thinking that qemu
updates the memslots or something, or maybe because guest memory
isn't fully migrated and relies on post-copy to finish it.
Also I am not against leaving CR3 processing in here and doing only PDPTR load
in KVM_RUN (and only when *SREG2 API is not used).
>
> diff --git a/x86/vmx_tests.c b/x86/vmx_tests.c
> index bbb006a..b8ccc69 100644
> --- a/x86/vmx_tests.c
> +++ b/x86/vmx_tests.c
> @@ -8172,6 +8172,16 @@ static void test_guest_segment_base_addr_fields(void)
> vmcs_write(GUEST_AR_ES, ar_saved);
> }
>
> +static void test_guest_cr3(void)
> +{
> + u64 cr3_saved = vmcs_read(GUEST_CR3);
> +
> + vmcs_write(GUEST_CR3, -1ull);
> + test_guest_state("Bad CR3 fails VM-Enter", true, -1ull, "GUEST_CR3");
> +
> + vmcs_write(GUEST_DR7, cr3_saved);
> +}
> +
Could you send this test to kvm unit tests?
> /*
> * Check that the virtual CPU checks the VMX Guest State Area as
> * documented in the Intel SDM.
> @@ -8181,6 +8191,8 @@ static void vmx_guest_state_area_test(void)
> vmx_set_test_stage(1);
> test_set_guest(guest_state_test_main);
>
> + test_guest_cr3();
> +
> /*
> * The IA32_SYSENTER_ESP field and the IA32_SYSENTER_EIP field
> * must each contain a canonical address.
>
>
> > + return false;
> > +
> > if (nested_cpu_has2(vmcs12, SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES)) {
> > /*
> > * Translate L1 physical address to host physical
> > @@ -3357,6 +3357,10 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
> > }
> >
> > if (from_vmentry) {
> > + if (nested_vmx_load_cr3(vcpu, vmcs12->guest_cr3,
> > + nested_cpu_has_ept(vmcs12), &entry_failure_code))
>
> This alignment is messed up; it looks like two separate function calls.
Sorry about this, I see it now.
>
> > + goto vmentry_fail_vmexit_guest_mode;
> > +
> > failed_index = nested_vmx_load_msr(vcpu,
> > vmcs12->vm_entry_msr_load_addr,
> > vmcs12->vm_entry_msr_load_count);
> > --
> > 2.26.2
> >
Best regards,
Maxim Levitsky
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