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

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

On Wed, Mar 16, 2022 at 05:48:04PM +0000, David Edmondson wrote:
> On Wednesday, 2022-03-16 at 16:05:01 GMT, Daniel P. Berrangé wrote:
> 
> > 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
> 
> Which AMD CPU is in this host?

AMD EPYC 7302P


With regards,
Daniel
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