On Wed, 3 Apr 2019 14:03:21 +0200
David Hildenbrand <david@xxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > ---
> > 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.
