Re: [Qemu-devel] [PATCH RFC 0/3] s390x/tcg: LAP support using immediate TLB invalidation

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 29 Sep 2017 14:09:04 +0200
David Hildenbrand <david@redhat.com> wrote:

> On 29.09.2017 13:49, Cornelia Huck wrote:
> > On Wed, 27 Sep 2017 19:00:24 +0200
> > David Hildenbrand <david@redhat.com> wrote:
> >   
> >> Details about Low-Address Protection can be found in description of
> >> patch 1 and 2. It is basically a subpage protection of the first two
> >> pages of every address space (for which it is enabled).
> >>
> >> We can achieve this by simply directly invalidating the TLB entry and
> >> therefore forcing every write accesses onto these two pages into the slow
> >> path.
> >>
> >> With this patch, I can boot Linux just fine (which uses LAP). This also
> >> makes all related kvm-unit-tests that we have pass.
> >>
> >> The checks are working that good, that I discovered a STFL bug. STFL
> >> stores into the low addresses but low-address protection does explicitly
> >> not apply. The Linux kernel calls STFL while LAP is active. So without
> >> patch nr 3, booting Linux will fail. (this change is also part of a patch
> >> of my SMP series).  
> > 
> > I fear I have lost track a bit with all those patches floating around.
> > IIUC, patch 3 fixes a real bug that is only exposed by your LAP
> > changes. It used to be part of the stfl changes in v1 of your smp
> > series but is no longer in v2. So, is this a patch that can be applied
> > to current s390-next?
> >   
> 
> The SMP series is based on both, this series and the CPU cleanup series
> you already picked up.
> 
> You can apply Patch 3 with the following modified description (requested
> by Thomas):
> 
> 
> s390x/tcg: make STFL store into the lowcore
> 
> Using virtual memory access is wrong and will soon include low-address
> protection checks, which is to be bypassed for STFL.
> 
> STFL is a privileged instruction and using LowCore requires
> !CONFIG_USER_ONLY, so add the ifdef and move the declaration to the
> right place.
> 
> This was originally part of a bigger STFL(E) refactoring.
> 
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> Reviewed-by: Thomas Huth <thuth@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> 
> 
> The other two patches, I will resend once we know how to handle the TLB
> invalidation.

OK, thanks for the clarification!