Re: [PATCH 3/7] x86: Grant AMX permission for guest

[Paolo Bonzini <pbonzini@redhat.com>]

On 1/27/22 14:45, Yang Zhong wrote:
>    
>    Thanks Paolo, from your below KVM changes:
>    https://lore.kernel.org/kvm/20220126152210.3044876-3-pbonzini@redhat.com/T/#m7bf9a03c47c29d21deb78604bc290a45aa5e98f5
> 
>    So the changes in kvm_arch_get_supported_cpuid() like below?
>    +    } else if (function == 0xd && index == 0 && reg == R_EAX) {

&& (reg == R_EAX || reg == R_EDX)

>    +	struct kvm_device_attr attr = {
>    +		.group = 0,
>    +		.attr = KVM_X86_XCOMP_GUEST_SUPP,
>    +		.addr = (unsigned long) &bitmask
>    +	};
>    +
>    +	kvm_fd = open_kvm_dev_path_or_exit();
>    +	rc = ioctl(kvm_fd, KVM_GET_DEVICE_ATTR, &attr);

Yes, you need to check if it fails though (and in that case do not 
assign).  The file descriptor is already there in QEMU, so you can use 
kvm_ioctl.

>   +     ret = bitmask;

To support R_EDX as well:

ret = (reg == R_EAX) ? bitmask : bitmask >> 32;

>    So in the kvm_request_xsave_components(), we can do below steps?
>   
>    +    /* Check supported_xr0 firstly */
>    +    rc = kvm_arch_get_supported_cpuid(s, 0xd, 0, R_EAX);
>    +    if (!(rc & XFEATURE_XTILE_MASK)) {

rc = kvm_arch_get_supported_cpuid(s, 0xd, 0,
                                   (xdata_bit < 32 ? R_EAX : R_EDX));
if (!(rc & BIT(xdata_bit & 31)) {
     ...
}

Also please use warn_report and don't exit.  With e.g. "-cpu 
foo,amx,xfd,enforce" you should not reach this code at all if the host 
lacks AMX.

Thanks,

Paolo

>    +        error_report("host xcr0 can't support AMX xdata and rc=0x%lx", rc);
>    +        exit(EXIT_FAILURE);
>    +    }
> 
>    +   /* request amx permission */
>    +   syscall(ARCH_REQ_XCOMP_GUEST_PERM, xdata_bit);
> 
> 
>    +   /* check amx permission */
>    +   syscall(ARCH_GET_XCOMP_GUEST_PERM);