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

[Michal Prívozník]

On 3/16/22 16:57, 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:
> 
> $ ~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.
> 
> 

Not trying to beat a dead horse here, but I've just found another
problem with this patch. On my laptop (Linux maggie
5.15.26-gentoo-x86_64 #1 SMP Thu Mar 10 08:55:28 CET 2022 x86_64
Intel(R) Core(TM) i7-10610U CPU @ 1.80GHz GenuineIntel GNU/Linux), when
I start a guest it no longer sees AVX instructions:

  qemu.git $ ./build/qemu-system-x86_64 -accel kvm -cpu host ...

Michal