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From:   Jim Mattson <jmattson@google.com>
Date:   Wed, 10 Jun 2020 10:11:38 -0700
Message-ID: <CALMp9eRzMC=6hFUeDP9V_CnJ29EbrC6KzWNXzbAsq7Uqqr=f6g@mail.gmail.com>
Subject: Re: [PATCH] KVM: selftests: fix sync_with_host() in smm_test
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     kvm list <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Marcelo Bandeira Condotta <mcondotta@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Jun 10, 2020 at 9:41 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
> It was reported that older GCCs compile smm_test in a way that breaks
> it completely:
>
>   kvm_exit:             reason EXIT_CPUID rip 0x4014db info 0 0
>   func 7ffffffd idx 830 rax 0 rbx 0 rcx 0 rdx 0, cpuid entry not found
>   ...
>   kvm_exit:             reason EXIT_MSR rip 0x40abd9 info 0 0
>   kvm_msr:              msr_read 487 = 0x0 (#GP)
>   ...
>
> Note, '7ffffffd' was supposed to be '80000001' as we're checking for
> SVM. Dropping '-O2' from compiler flags help. Turns out, asm block in
> sync_with_host() is wrong. We us 'in 0xe, %%al' instruction to sync
> with the host and in 'AL' register we actually pass the parameter
> (stage) but after sync 'AL' gets written to but GCC thinks the value
> is still there and uses it to compute 'EAX' for 'cpuid'.

That smells like VMware's hypercall madness!

> smm_test can't fully use standard ucall() framework as we need to
> write a very simple SMI handler there. Fix the immediate issue by
> making RAX input/output operand. While on it, make sync_with_host()
> static inline.
>
> Reported-by: Marcelo Bandeira Condotta <mcondotta@redhat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Jim Mattson <jmattson@google.com>