From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Kalra, Ashish" <Ashish.Kalra@cavium.com>
Subject: Re: [kvm-unit-tests PATCH] arm64: add micro test
Date: Thu, 3 May 2018 23:38:30 +0530
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Cc: pbonzini@redhat.com,
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To: Marc Zyngier <marc.zyngier@arm.com>, Andrew Jones <drjones@redhat.com>,
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 >>On 5/3/2018 9:54 PM, Marc Zyngier wrote:
>> On 03/05/18 12:12, Kalra, Ashish wrote:
>>>
>>>>>
>>>> The reason for the problem you're seeing is probably that the
>>>>> constraints are not quite right. Here, x0 is not simply clobbered, but
>>>>> is actively written to in the middle of the sequence (it is at least an
>>>>> early clobber). It is also, I assume, a result from the hypercall, so it
>>>>> cannot simply be discarded.
>>>>>
>>>>> It would help to get a disassembly of the function, but I'd tend to
>>>>> rewrite the code as such:
>>>>>
>>>>> extern int bar(void);
>>>>>
>>>>> int foo(void)
>>>>> {
>>>>> 	register int w0 asm("w0");
>>>>> 	int a, b;
>>>>>
>>>>> 	a = bar();
>>>>> 	w0 = 0x4b000000;
>>>>> 	asm volatile("hvc #0" : "+r" (w0) :: );
>>>>> 	b = bar();
>>>>>
>>>>> 	return a - b;
>>>>> }
>>>
>>> There still is the issue of x1-x3 registers being used for local
>>> variables and clobbering of the same across the "hvc" call.
>>>
>>> Comparatively, the fix below works more reliably :
>>>
>>>> asm volatile("hvc #0" : "+r" (w0) :: "x1","x2","x3");
>> 
>> Is that because this hypercall is explicitly clobbering these registers?
>> or because KVM is corrupting them? If the former, fine. If the latter,
>> that'd be a KVM bug.
>> 
>> So which one is it?
>> 

It is being explicitly done by the hypercall handler:

int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
{
	u32 func_id = smccc_get_function(vcpu);
         u32 val = PSCI_RET_NOT_SUPPORTED;
         ..
         ..
         smccc_set_retval(vcpu, val, 0, 0, 0);
}

Thanks,
Ashish