linux-kernel - Re: [PATCH] KVM: selftests: fix sync_with_host() in smm

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Date:   Thu, 11 Jun 2020 01:54:21 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Marcelo Bandeira Condotta <mcondotta@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: selftests: fix sync_with_host() in smm_test

On 10/06/20 18:41, Vitaly Kuznetsov 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'.
> 
> 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@...hat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  tools/testing/selftests/kvm/x86_64/smm_test.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/x86_64/smm_test.c b/tools/testing/selftests/kvm/x86_64/smm_test.c
> index 36314152943d..ae39a220609f 100644
> --- a/tools/testing/selftests/kvm/x86_64/smm_test.c
> +++ b/tools/testing/selftests/kvm/x86_64/smm_test.c
> @@ -47,10 +47,10 @@ uint8_t smi_handler[] = {
>  	0x0f, 0xaa,           /* rsm */
>  };
>  
> -void sync_with_host(uint64_t phase)
> +static inline void sync_with_host(uint64_t phase)
>  {
>  	asm volatile("in $" XSTR(SYNC_PORT)", %%al \n"
> -		     : : "a" (phase));
> +		     : "+a" (phase));
>  }
>  
>  void self_smi(void)
> 

Queued, thanks.

Paolo