linux-kernel - Re: [PATCH 1/2] KVM: selftests: Implement ucall() for s390x

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Date:   Wed, 31 Jul 2019 11:43:16 +0200
From:   Thomas Huth <thuth@...hat.com>
To:     Andrew Jones <drjones@...hat.com>
Cc:     kvm@...r.kernel.org,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Shuah Khan <shuah@...nel.org>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH 1/2] KVM: selftests: Implement ucall() for s390x

On 30/07/2019 12.48, Andrew Jones wrote:
> On Tue, Jul 30, 2019 at 12:01:11PM +0200, Thomas Huth wrote:
>> On s390x, we can neither exit via PIO nor MMIO, but have to use
>> an instruction like DIAGNOSE. While we're at it, rename UCALL_PIO
>> to UCALL_DEFAULT, since PIO only works on x86 anyway, and this
>> way we can re-use the "default" type for the DIAGNOSE exit on s390x.
>>
>> Now that ucall() is implemented, we can use it in the sync_reg_test
>> on s390x, too.
>>
>> Signed-off-by: Thomas Huth <thuth@...hat.com>
>> ---
>>  .../testing/selftests/kvm/include/kvm_util.h  |  2 +-
>>  tools/testing/selftests/kvm/lib/ucall.c       | 34 +++++++++++++++----
>>  .../selftests/kvm/s390x/sync_regs_test.c      |  6 ++--
>>  3 files changed, 32 insertions(+), 10 deletions(-)
>>
>> diff --git a/tools/testing/selftests/kvm/include/kvm_util.h b/tools/testing/selftests/kvm/include/kvm_util.h
>> index e0e66b115ef2..c37aea2e33e5 100644
>> --- a/tools/testing/selftests/kvm/include/kvm_util.h
>> +++ b/tools/testing/selftests/kvm/include/kvm_util.h
>> @@ -167,7 +167,7 @@ int vm_create_device(struct kvm_vm *vm, struct kvm_create_device *cd);
>>  
>>  /* ucall implementation types */
>>  typedef enum {
>> -	UCALL_PIO,
>> +	UCALL_DEFAULT,
> 
> I'd rather we keep explicit types defined; keep PIO and add DIAG. Then
> we can have
> 
> /*  Set default ucall types */
> #if defined(__x86_64__)
>   ucall_type = UCALL_PIO;
> #elif defined(__aarch64__)
>   ucall_type = UCALL_MMIO;
>   ucall_requires_init = true;
> #elif defined(__s390x__)
>   ucall_type = UCALL_DIAG;
> #endif
> 
> And add an assert in get_ucall()
> 
>  assert(!ucall_requires_init || ucall_initialized);

I'm not sure whether I really like that. It's yet another additional
#ifdef block, and yet another variable ...

What do you think about removing the enum completely and simply code it
directly, without the ucall_type indirection, i.e.:

void ucall(uint64_t cmd, int nargs, ...)
{
	struct ucall uc = {
		.cmd = cmd,
	};
	va_list va;
	int i;

	nargs = nargs <= UCALL_MAX_ARGS ? nargs : UCALL_MAX_ARGS;

	va_start(va, nargs);
	for (i = 0; i < nargs; ++i)
		uc.args[i] = va_arg(va, uint64_t);
	va_end(va);

#if defined(__x86_64__)

	/* Exit via PIO */
	asm volatile("in %[port], %%al"
		: : [port] "d" (UCALL_PIO_PORT), "D" (&uc) : "rax");

#elif defined(__aarch64__)

	*ucall_exit_mmio_addr = (vm_vaddr_t)&uc;

#elif defined(__s390x__)

	/* Exit via DIAGNOSE 0x501 (normally used for breakpoints) */
	asm volatile ("diag 0,%0,0x501" : : "a"(&uc) : "memory");

#endif
}

I think that's way less confusing than having to understand the meaning
of ucall_type etc. before...?

 Thomas