linux-kernel - Re: [PATCH v3 2/8] x86/sev: Add support for the RMPREAD instruction

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Message-ID: <7820cd89-f25a-4934-9597-d53e861d6e92@amd.com>
Date: Fri, 18 Oct 2024 10:14:04 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Borislav Petkov <bp@...en8.de>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Michael Roth <michael.roth@....com>, Ashish Kalra <ashish.kalra@....com>
Subject: Re: [PATCH v3 2/8] x86/sev: Add support for the RMPREAD instruction

On 10/18/24 07:41, Borislav Petkov wrote:
> On Mon, Sep 30, 2024 at 10:22:10AM -0500, Tom Lendacky wrote:
>> diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
>> index 103a2dd6e81d..73d4f422829a 100644
>> --- a/arch/x86/virt/svm/sev.c
>> +++ b/arch/x86/virt/svm/sev.c
>> @@ -301,6 +301,17 @@ static int get_rmpentry(u64 pfn, struct rmpentry *entry)
>>  {
>>  	struct rmpentry_raw *e;
>>  
>> +	if (cpu_feature_enabled(X86_FEATURE_RMPREAD)) {
>> +		int ret;
>> +
>> +		asm volatile(".byte 0xf2, 0x0f, 0x01, 0xfd"
>> +			     : "=a" (ret)
>> +			     : "a" (pfn << PAGE_SHIFT), "c" (entry)
>> +			     : "memory", "cc");
>> +
>> +		return ret;
>> +	}
> 
> I think this should be:
> 
> diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
> index 73d9295dd013..5500c5d64cba 100644
> --- a/arch/x86/virt/svm/sev.c
> +++ b/arch/x86/virt/svm/sev.c
> @@ -303,12 +303,11 @@ static int get_rmpentry(u64 pfn, struct rmpentry *entry)
>  	struct rmpentry_raw *e;
>  
>  	if (cpu_feature_enabled(X86_FEATURE_RMPREAD)) {
> -		int ret;
> +		int ret = pfn << PAGE_SHIFT;
>  
>  		asm volatile(".byte 0xf2, 0x0f, 0x01, 0xfd"
> -			     : "=a" (ret)
> -			     : "a" (pfn << PAGE_SHIFT), "c" (entry)
> -			     : "memory", "cc");
> +			     : "+a" (ret), "+c" (entry)
> +			     :: "memory", "cc");
>  
>  		return ret;
>  	}
> 
> because "The RCX register provides the effective address of a 16-byte data
> structure into which the RMP state is written."
> 
> So your %rcx is both an input and an output operand and you need to do the "+"
> thing here too for that.

I don't think so. RCX does not change on output, the contents that RCX
points to changes, but the register value does not so the "+" is not
correct. The instruction doesn't take a memory location as part of
operands (like a MOV instruction could), which is why the "memory" clobber
is specified.

> 
> Same for %rax.

For RAX, yes, if I set "ret" to the input value then I can use "+"
specification. But the way it's coded now is also correct.

Thanks,
Tom

>