Re: [kvm-unit-tests PATCH v2 3/3] s390x: Ultravisor guest API test

[Cornelia Huck <cohuck@redhat.com>]

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On Fri, 31 Jul 2020 09:34:41 +0200
Janosch Frank <frankja@linux.ibm.com> wrote:

> On 7/30/20 5:58 PM, Thomas Huth wrote:
> > On 30/07/2020 13.16, Cornelia Huck wrote:  
> >> On Mon, 27 Jul 2020 05:54:15 -0400
> >> Janosch Frank <frankja@linux.ibm.com> wrote:
> >>  
> >>> Test the error conditions of guest 2 Ultravisor calls, namely:
> >>>      * Query Ultravisor information
> >>>      * Set shared access
> >>>      * Remove shared access
> >>>
> >>> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
> >>> Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> >>> ---
> >>>  lib/s390x/asm/uv.h  |  68 +++++++++++++++++++
> >>>  s390x/Makefile      |   1 +
> >>>  s390x/unittests.cfg |   3 +
> >>>  s390x/uv-guest.c    | 159 ++++++++++++++++++++++++++++++++++++++++++++
> >>>  4 files changed, 231 insertions(+)
> >>>  create mode 100644 lib/s390x/asm/uv.h
> >>>  create mode 100644 s390x/uv-guest.c
> >>>  
> >>
> >> (...)
> >>  
> >>> +static inline int uv_call(unsigned long r1, unsigned long r2)
> >>> +{
> >>> +	int cc;
> >>> +
> >>> +	asm volatile(
> >>> +		"0:	.insn rrf,0xB9A40000,%[r1],%[r2],0,0\n"
> >>> +		"		brc	3,0b\n"
> >>> +		"		ipm	%[cc]\n"
> >>> +		"		srl	%[cc],28\n"
> >>> +		: [cc] "=d" (cc)
> >>> +		: [r1] "a" (r1), [r2] "a" (r2)
> >>> +		: "memory", "cc");
> >>> +	return cc;
> >>> +}  
> >>
> >> This returns the condition code, but no caller seems to check it
> >> (instead, they look at header.rc, which is presumably only set if the
> >> instruction executed successfully in some way?)
> >>
> >> Looking at the kernel, it retries for cc > 1 (presumably busy
> >> conditions), and cc != 0 seems to be considered a failure. Do we want
> >> to look at the cc here as well?  
> > 
> > It's there - but here it's in the assembly code, the "brc 3,0b".  

Ah yes, I missed that.

> 
> Yes, we needed to factor that out in KVM because we sometimes need to
> schedule and then it looks nicer handling that in C code. The branch on
> condition will jump back for cc 2 and 3. cc 0 and 1 are success and
> error respectively and only then the rc and rrc in the UV header are set.

Yeah, it's a bit surprising that rc/rrc are also set with cc 1.

(Can you add a comment? Just so that it is clear that callers never
need to check the cc, as rc/rrc already contain more information than
that.)

> 
> > 
> > Patch looks ok to me (but I didn't do a full review):
> > 
> > Acked-by: Thomas Huth <thuth@redhat.com>
> >   
> 
> 


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