Re: [PULL 15/22] x86: Grant AMX permission for guest

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Mar 16, 2022 at 04:57:39PM +0100, Peter Krempa wrote:
> On Tue, Mar 08, 2022 at 12:34:38 +0100, Paolo Bonzini wrote:
> > From: Yang Zhong <yang.zhong@intel.com>
> > 
> > Kernel allocates 4K xstate buffer by default. For XSAVE features
> > which require large state component (e.g. AMX), Linux kernel
> > dynamically expands the xstate buffer only after the process has
> > acquired the necessary permissions. Those are called dynamically-
> > enabled XSAVE features (or dynamic xfeatures).
> > 
> > There are separate permissions for native tasks and guests.
> > 
> > Qemu should request the guest permissions for dynamic xfeatures
> > which will be exposed to the guest. This only needs to be done
> > once before the first vcpu is created.
> > 
> > KVM implemented one new ARCH_GET_XCOMP_SUPP system attribute API to
> > get host side supported_xcr0 and Qemu can decide if it can request
> > dynamically enabled XSAVE features permission.
> > https://lore.kernel.org/all/20220126152210.3044876-1-pbonzini@redhat.com/
> > 
> > Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> > Signed-off-by: Yang Zhong <yang.zhong@intel.com>
> > Signed-off-by: Jing Liu <jing2.liu@intel.com>
> > Message-Id: <20220217060434.52460-4-yang.zhong@intel.com>
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > ---
> >  target/i386/cpu.c          |  7 +++++
> >  target/i386/cpu.h          |  4 +++
> >  target/i386/kvm/kvm-cpu.c  | 12 ++++----
> >  target/i386/kvm/kvm.c      | 57 ++++++++++++++++++++++++++++++++++++++
> >  target/i386/kvm/kvm_i386.h |  1 +
> >  5 files changed, 75 insertions(+), 6 deletions(-)
> 
> With this commit qemu crashes for me when invoking the following
> QMP command:

It is way worse than that even. If you remove '-S' you get an
immediate kaboom on startup on AMD hosts

$ ./build/qemu-system-x86_64 -accel kvm
Unable to init server: Could not connect: Connection refused
qemu-system-x86_64: ../target/i386/kvm/kvm-cpu.c:105: kvm_cpu_xsave_init: Assertion `esa->size == eax' failed.
Aborted (core dumped)


With regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|