From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Wed, 3 Apr 2019 14:33:33 +0200 From: Cornelia Huck Subject: Re: [PATCH v3 1/2] s390/setup: diag318: remove bit check and refactor struct Message-ID: <20190403143333.2a3db681.cohuck@redhat.com> In-Reply-To: <599addd3-a4f8-d4e7-5898-dc45f52cd7a6@redhat.com> References: <20190402174636.15175-1-walling@linux.ibm.com> <20190402174636.15175-2-walling@linux.ibm.com> <599addd3-a4f8-d4e7-5898-dc45f52cd7a6@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-doc-owner@vger.kernel.org List-Archive: List-Post: To: David Hildenbrand Cc: Collin Walling , pbonzini@redhat.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, schwidefsky@de.ibm.com, heiko.carstens@de.ibm.com, borntraeger@de.ibm.com, frankja@linux.ibm.com List-ID: On Wed, 3 Apr 2019 14:03:21 +0200 David Hildenbrand 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 > > --- > > 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.