Re: [PATCH 11/15] KVM: VMX: Update vmcs.GUEST_CR3 only when the guest CR3 is dirty

Maxim Levitsky
2021-12-15T20:21:08Z

On Thu, 2021-12-16 at 00:31 +0800, Lai Jiangshan wrote:
>
> On 2021/12/15 23:47, Maxim Levitsky wrote:
> > On Mon, 2021-11-08 at 20:44 +0800, Lai Jiangshan wrote:
> > > From: Lai Jiangshan <laijs@linux.alibaba.com>
> > >
> > > When vcpu->arch.cr3 is changed, it is marked dirty, so vmcs.GUEST_CR3
> > > can be updated only when kvm_register_is_dirty(vcpu, VCPU_EXREG_CR3).
> > >
> > > Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> > > ---
> > > arch/x86/kvm/vmx/vmx.c | 4 ++--
> > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > > index d94e51e9c08f..38b65b97fb7b 100644
> > > --- a/arch/x86/kvm/vmx/vmx.c
> > > +++ b/arch/x86/kvm/vmx/vmx.c
> > > @@ -3126,9 +3126,9 @@ static void vmx_load_mmu_pgd(struct kvm_vcpu *vcpu, hpa_t root_hpa,
> > >
> > > if (!enable_unrestricted_guest && !is_paging(vcpu))
> > > guest_cr3 = to_kvm_vmx(kvm)->ept_identity_map_addr;
> > > - else if (test_bit(VCPU_EXREG_CR3, (ulong *)&vcpu->arch.regs_avail))
> > > + else if (kvm_register_is_dirty(vcpu, VCPU_EXREG_CR3))
> > > guest_cr3 = vcpu->arch.cr3;
> > > - else /* vmcs01.GUEST_CR3 is already up-to-date. */
> > > + else /* vmcs.GUEST_CR3 is already up-to-date. */
> > > update_guest_cr3 = false;
> > > vmx_ept_load_pdptrs(vcpu);
> > > } else {
> >
> > I just bisected this patch to break booting a VM with ept=1 but unrestricted_guest=0
> > (I needed to re-test unrestricted_guest=0 bug related to SMM, but didn't want
> > to boot without EPT. With ept=0,the VM boots with this patch applied).
> >
>
> Thanks for reporting.
>
> Sorry, I never tested it with unrestricted_guest=0. I can't reproduce it now shortly
> with unrestricted_guest=0. Maybe it can be reproduced easily if I try more guests or
> I write a piece of guest code to deliberate hit it if the following analyses is correct.
>
> All the paths changing %cr3 are followed with kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3)
> and GUEST_CR3 will be expected to be updated.
>
> What I missed is the case of "if (!enable_unrestricted_guest && !is_paging(vcpu))"
> in vmx_load_mmu_pgd() which doesn't load GUEST_CR3 but clears dirty of VCPU_EXREG_CR3
> (when after next run).
>
> So when CR0 !PG -> PG, VCPU_EXREG_CR3 dirty bit should be set.
>
> Maybe adding the following patch on top of the original patch can work.
>
> Thanks
> Lai
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 85127b3e3690..55b45005ebb9 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -858,6 +858,7 @@ void kvm_post_set_cr0(struct kvm_vcpu *vcpu, unsigned long old_cr0, unsigned lon
> if ((cr0 ^ old_cr0) & X86_CR0_PG) {
> kvm_clear_async_pf_completion_queue(vcpu);
> kvm_async_pf_hash_reset(vcpu);
> + kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
> }
>
> if ((cr0 ^ old_cr0) & KVM_MMU_CR0_ROLE_BITS)
>

Tested this patch and my guests boot. I didn't test more stuff like migration or so,
will do tomorrow.

Best regards,
Maxim Levitsky