Re: [PATCH] x86/kvm/vmx: don't read current->thread.{fs,gs}base of legacy tasks

Vitaly Kuznetsov
2018-07-12T11:23:15Z

Vitaly Kuznetsov <vkuznets@redhat.com> writes:

> Wanpeng Li <kernellwp@gmail.com> writes:
>
>> Test suite: MSR switch
>> PASS: VM entry MSR load
>> PASS: VM exit MSR store
>> PASS: VM exit MSR load
>> FAIL: VM entry MSR load: try to load FS_BASE
>> SUMMARY: 4 tests, 1 unexpected failures
>>
>> kvm-unit-tests fails w/ and w/o the patch, maybe it is another issue,
>> i didn't dig further, you can have a look if you are interested in. :)
>
> The patch only changes the behavior for legacy userspaces and I can
> reproduce the failure on native x86_64, it is something different. I'm,
> however, interested so stay tuned :-)

Yes,

the regression was introduced by

commit e79f245ddec17bbd89d73cd0169dba4be46c9b55
Author: KarimAllah Ahmed <karahmed@amazon.de>
Date: Sat Apr 14 05:10:52 2018 +0200

X86/KVM: Properly update 'tsc_offset' to represent the running guest

basically, when nested_vmx_load_msr() fails we don't set
exit_qualification accordingly.

The fix is simple:

@@ -11720,8 +11721,10 @@ static int enter_vmx_non_root_mode(struct kvm_vcpu *vcpu)
msr_entry_idx = nested_vmx_load_msr(vcpu,
vmcs12->vm_entry_msr_load_addr,
vmcs12->vm_entry_msr_load_count);
- if (msr_entry_idx)
+ if (msr_entry_idx) {
+ exit_qual = msr_entry_idx;
goto fail;
+ }

/*
* Note no nested_vmx_succeed or nested_vmx_fail here. At this point

I'll be sending a patch out shortly. But this is completely orthogonal
to the 'legacy' issue ;-)

--
Vitaly