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

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Date:   Thu, 12 Jul 2018 13:23:09 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        "the arch\/x86 maintainers" <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>, ldv@...linux.org,
        yamato@...hat.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/kvm/vmx: don't read current->thread.{fs,gs}base of legacy tasks

Vitaly Kuznetsov <vkuznets@...hat.com> writes:

> Wanpeng Li <kernellwp@...il.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@...zon.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