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

[Peter Krempa]

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:

$ ~pipo/git/qemu.git/build/qemu-system-x86_64 -S -no-user-config -nodefaults 
-nographic -machine none,accel=kvm -qmp stdio
{"QMP": {"version": {"qemu": {"micro": 90, "minor": 2, "major": 6}, "package": 
"v7.0.0-rc0-8-g1d60bb4b14"}, "capabilities": ["oob"]}}
{'execute':'qmp_capabilities'}
{"return": {}}
{"execute":"qom-list-properties","arguments":{"typename":"max-x86_64-cpu"},"id":"libvirt-41"}
qemu-system-x86_64: ../target/i386/kvm/kvm-cpu.c:105: kvm_cpu_xsave_init: 
Assertion `esa->size == eax' failed.
Aborted (core dumped)

Note that the above is on a box with an 'AMD Ryzen 9 3900X'.

Curiously on a laptop with an Intel chip (Intel(R) Core(TM) i7-10610U)
it seems to work.