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

[Yang Zhong]

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:
> 
> $ ~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.

  Thanks for pointing this out!
  
  In my side, no AMD machine can be used to try this issue, I listed the
  FPU info from host kernel dmesg for reference.
  
  root@984fee00bf64:~# dmesg | grep fpu
  [    0.000000] x86/fpu: Supporting XSAVE feature 0x001: 'x87 floating point 
registers'
  [    0.000000] x86/fpu: Supporting XSAVE feature 0x002: 'SSE registers'
  [    0.000000] x86/fpu: Supporting XSAVE feature 0x004: 'AVX registers'
  [    0.000000] x86/fpu: Supporting XSAVE feature 0x020: 'AVX-512 opmask'
  [    0.000000] x86/fpu: Supporting XSAVE feature 0x040: 'AVX-512 Hi256'
  [    0.000000] x86/fpu: Supporting XSAVE feature 0x080: 'AVX-512 ZMM_Hi256'
  [    0.000000] x86/fpu: Supporting XSAVE feature 0x200: 'Protection Keys User 
registers'
  [    0.000000] x86/fpu: Supporting XSAVE feature 0x400: 'PASID state'
  [    0.000000] x86/fpu: Supporting XSAVE feature 0x20000: 'AMX Tile config'
  [    0.000000] x86/fpu: Supporting XSAVE feature 0x40000: 'AMX Tile data'
  [    0.000000] x86/fpu: xstate_offset[2]:  576, xstate_sizes[2]:  256
  [    0.000000] x86/fpu: xstate_offset[5]:  832, xstate_sizes[5]:   64
  [    0.000000] x86/fpu: xstate_offset[6]:  896, xstate_sizes[6]:  512
  [    0.000000] x86/fpu: xstate_offset[7]: 1408, xstate_sizes[7]: 1024
  [    0.000000] x86/fpu: xstate_offset[9]: 2432, xstate_sizes[9]:    8
  [    0.000000] x86/fpu: xstate_offset[10]: 2440, xstate_sizes[10]:    8
  [    0.000000] x86/fpu: xstate_offset[17]: 2496, xstate_sizes[17]:   64
  [    0.000000] x86/fpu: xstate_offset[18]: 2560, xstate_sizes[18]: 8192
  [    0.000000] x86/fpu: Enabled xstate features 0x606e7, context size is 
10752 bytes, using 'compacted' format.

  Paolo, if you have fix patch, I can double check this from Intel SPR server. 
thanks!

  Yang