linux-kernel - Re: [PATCH v3 1/2] s390/setup: diag318: remove bit check and refactor struct

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Date:   Wed, 3 Apr 2019 14:33:33 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Collin Walling <walling@...ux.ibm.com>, pbonzini@...hat.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, borntraeger@...ibm.com,
        frankja@...ux.ibm.com
Subject: Re: [PATCH v3 1/2] s390/setup: diag318: remove bit check and
 refactor struct

On Wed, 3 Apr 2019 14:03:21 +0200
David Hildenbrand <david@...hat.com> wrote:

> On 02.04.19 19:46, Collin Walling wrote:
> > Execution of DIAGNOSE 0x318 is fenced by checking an SCLP bit
> > for the availability of hardware support for the instruction.
> > 
> > In order to support this instruction for a KVM/QEMU guest, we
> > would need to provide modifications to the SCLP Read SCP Info
> > data, which will in turn reduce the maximum number of CPUs that
> > may be provided to the guest. This issue introduces compatability
> > and legacy concerns.
> > 
> > Let's circumvent this issue by removing the bit check and blindly
> > executing the instruction. An exception table rule is in place to
> > catch the case where hardware does not support this instruction.
> > 
> > While we're at it, let's condense the version code fields in the
> > diag318_info struct until we can determine how it will be used.
> > 
> > This modifies commit 4ad78b8651aacf26b3ab6d1e784952eb70469c43
> > 
> > Signed-off-by: Collin Walling <walling@...ux.ibm.com>
> > ---
> >  arch/s390/include/asm/diag.h |  6 ++----
> >  arch/s390/kernel/setup.c     | 12 ++++++------
> >  2 files changed, 8 insertions(+), 10 deletions(-)
> > 
> > diff --git a/arch/s390/include/asm/diag.h b/arch/s390/include/asm/diag.h
> > index 19562be22b7e..215516284175 100644
> > --- a/arch/s390/include/asm/diag.h
> > +++ b/arch/s390/include/asm/diag.h
> > @@ -298,10 +298,8 @@ struct diag26c_mac_resp {
> >  union diag318_info {
> >  	unsigned long val;
> >  	struct {
> > -		unsigned int cpnc : 8;
> > -		unsigned int cpvc_linux : 24;
> > -		unsigned char cpvc_distro[3];
> > -		unsigned char zero;
> > +		unsigned long cpnc : 8;
> > +		unsigned long cpvc : 56;

That part looks reasonable (we don't have a proper convention yet, have
we?)

> >  	};
> >  };
> >  
> > diff --git a/arch/s390/kernel/setup.c b/arch/s390/kernel/setup.c
> > index 2c642af526ce..fe70201f8b5d 100644
> > --- a/arch/s390/kernel/setup.c
> > +++ b/arch/s390/kernel/setup.c
> > @@ -1011,15 +1011,15 @@ static void __init setup_control_program_code(void)
> >  {
> >  	union diag318_info diag318_info = {
> >  		.cpnc = CPNC_LINUX,
> > -		.cpvc_linux = 0,
> > -		.cpvc_distro = {0},
> > +		.cpvc = 0,
> >  	};
> >  
> > -	if (!sclp.has_diag318)
> > -		return;
> > -
> >  	diag_stat_inc(DIAG_STAT_X318);
> > -	asm volatile("diag %0,0,0x318\n" : : "d" (diag318_info.val));
> > +	asm volatile(
> > +		"	diag	%0,0,0x318\n"
> > +		"0:	nopr	%%r7\n"
> > +		EX_TABLE(0b,0b)
> > +		: : "d" (diag318_info.val));
> >  }
> >  
> >  /*
> >   
> 
> That smells like a nasty hack to not expose new features in QEMU and
> deal with the issue of handling CPU limits. No, I don't like this.
> 
> Fix QEMU, not the kernel.
> 

I agree. The compat handling is a bit annoying, but I don't think we
can get around it.