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Subject: Re: [RFC V2 PATCH 1/8] selftests: kvm: Fix inline assembly for
 hypercall
To:     Vishal Annapurve <vannapurve@google.com>, x86@kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org
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        mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        hpa@zytor.com, shauh@kernel.org, yang.zhong@intel.com,
        drjones@redhat.com, ricarkol@google.com, aaronlewis@google.com,
        wei.w.wang@intel.com, kirill.shutemov@linux.intel.com,
        corbet@lwn.net, hughd@google.com, jlayton@kernel.org,
        bfields@fieldses.org, akpm@linux-foundation.org,
        chao.p.peng@linux.intel.com, yu.c.zhang@linux.intel.com,
        jun.nakajima@intel.com, dave.hansen@intel.com,
        michael.roth@amd.com, qperret@google.com, steven.price@arm.com,
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        vbabka@suse.cz, marcorr@google.com, erdemaktas@google.com,
        pgonda@google.com, nikunj@amd.com, seanjc@google.com,
        diviness@google.com, Shuah Khan <skhan@linuxfoundation.org>
References: <20220511000811.384766-1-vannapurve@google.com>
 <20220511000811.384766-2-vannapurve@google.com>
From:   Shuah Khan <skhan@linuxfoundation.org>
Message-ID: <506a2e59-eea5-8c1e-ee1b-fbb7a401bd8d@linuxfoundation.org>
Date:   Thu, 12 May 2022 11:31:43 -0600
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On 5/10/22 6:08 PM, Vishal Annapurve wrote:
> Fix inline assembly for hypercall to explicitly set
> eax with hypercall number to allow the implementation
> to work even in cases where compiler would inline the
> function.
> 

Please explain what happens without this change as well.

> Signed-off-by: Vishal Annapurve <vannapurve@google.com>
> ---
>   tools/testing/selftests/kvm/lib/x86_64/processor.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
> index 9f000dfb5594..4d88e1a553bf 100644
> --- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
> +++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
> @@ -1461,7 +1461,7 @@ uint64_t kvm_hypercall(uint64_t nr, uint64_t a0, uint64_t a1, uint64_t a2,
>   
>   	asm volatile("vmcall"
>   		     : "=a"(r)
> -		     : "b"(a0), "c"(a1), "d"(a2), "S"(a3));
> +		     : "a"(nr), "b"(a0), "c"(a1), "d"(a2), "S"(a3));
>   	return r;
>   }
>   
> 

With the above change to commit log:

Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>

thanks,
-- Shuah