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

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Date:   Wed, 31 Jul 2019 12:28:49 +0200
From:   Andrew Jones <drjones@...hat.com>
To:     Thomas Huth <thuth@...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 Wed, Jul 31, 2019 at 11:43:16AM +0200, Thomas Huth wrote:
> 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...?
>

Sounds good to me.

Thanks,
drew