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

Lai Jiangshan
2021-12-15T16:31:24Z

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)