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Re: [PATCH RFC v1 2/2] i386: reorder kvm_put_sregs2() and kvm_put_nested
From: |
Vitaly Kuznetsov |
Subject: |
Re: [PATCH RFC v1 2/2] i386: reorder kvm_put_sregs2() and kvm_put_nested_state() when vCPU is reset |
Date: |
Wed, 10 Aug 2022 16:57:19 +0200 |
Maxim Levitsky <mlevitsk@redhat.com> writes:
> On Wed, 2022-08-10 at 16:00 +0200, Vitaly Kuznetsov wrote:
>> Setting nested state upon migration needs to happen after kvm_put_sregs2()
>> to e.g. have EFER.SVME set. This, however, doesn't work for vCPU reset:
>> when vCPU is in VMX root operation, certain CR bits are locked and
>> kvm_put_sregs2() may fail. As nested state is fully cleaned up upon
>> vCPU reset (kvm_arch_reset_vcpu() -> kvm_init_nested_state()), calling
>> kvm_put_nested_state() before kvm_put_sregs2() is OK, this will ensure
>> that vCPU is *not* in VMX root opertaion.
>>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>> target/i386/kvm/kvm.c | 20 ++++++++++++++++++--
>> 1 file changed, 18 insertions(+), 2 deletions(-)
>>
>> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
>> index 4f8dacc1d4b5..73e3880fa57b 100644
>> --- a/target/i386/kvm/kvm.c
>> +++ b/target/i386/kvm/kvm.c
>> @@ -4529,18 +4529,34 @@ int kvm_arch_put_registers(CPUState *cpu, int level)
>>
>> assert(cpu_is_stopped(cpu) || qemu_cpu_is_self(cpu));
>>
>> - /* must be before kvm_put_nested_state so that EFER.SVME is set */
>> + /*
>> + * When resetting a vCPU, make sure to reset nested state first to
>> + * e.g clear VMXON state and unlock certain CR4 bits.
>> + */
>> + if (level == KVM_PUT_RESET_STATE) {
>> + ret = kvm_put_nested_state(x86_cpu);
>> + if (ret < 0) {
>> + return ret;
>> + }
>
> I should have mentioned this, I actually already debugged the same issue while
> trying to reproduce the smm int window bug.
> 100% my fault.
>
> I also share the same feeling that this might be yet another 'whack a mole'
> and
> break somewhere else, but overall it does make sense.
This certainly *is* a 'whack a mole' and I'm sure there are other cases
when one of calls in kvm_arch_put_registers() fails. We need to work on
what's missing so we can expose kvm_vcpu_reset() to VMMs.
>
>
> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
>
Thanks!
--
Vitaly