Re: [PATCH v3 01/13] x86/fpu/xstate: Avoid getting xstate address of init_fpstate if fpstate contains the component

[Chang S. Bae]

On 2/21/2023 8:36 AM, Mingwei Zhang wrote:
> Avoid getting xstate address of init_fpstate if fpstate contains the xstate
> component. Since XTILEDATA (bit 18) was turned off in xinit, when KVM calls
> __raw_xsave_addr(xinit, 18), it triggers a warning as follows.
>
> __raw_xsave_addr() is an internal function that assume caller does the
> checking, ie., all function arguments should be checked before calling.
> So, instead of removing the WARNING, add checks in
> __copy_xstate_to_uabi_buf().

<snip>

> @@ -1151,10 +1152,11 @@ void __copy_xstate_to_uabi_buf(struct membuf to, struct fpstate *fpstate,
>   			pkru.pkru = pkru_val;
>   			membuf_write(&to, &pkru, sizeof(pkru));
>   		} else {
> -			copy_feature(header.xfeatures & BIT_ULL(i), &to,
> -				     __raw_xsave_addr(xsave, i),
> -				     __raw_xsave_addr(xinit, i),
> -				     xstate_sizes[i]);
> +			xsave_addr = (header.xfeatures & BIT_ULL(i)) ?
> +				__raw_xsave_addr(xsave, i) :
> +				__raw_xsave_addr(xinit, i);
> +
> +			membuf_write(&to, xsave_addr, xstate_sizes[i]);
>   		}
>   		/*
>   		 * Keep track of the last copied state in the non-compacted

So this hunk is under for_each_extended_xfeature(i, mask) -- it skips 
the copy routine if mask[i] == 0; instead, it fills zeros.

We have this [1]:

	if (fpu_state_size_dynamic())
		mask &= (header.xfeatures | xinit->header.xcomp_bv);

If header.xfeatures[18] = 0 then mask[18] = 0 because 
xinit->header.xcomp_bv[18] = 0. Then, it won't hit that code. So, I'm 
confused about the problem that you described here.

Can you elaborate on your test case a bit? Let me try to reproduce the 
issue on my end.

Thanks,
Chang

[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/fpu/xstate.c#n1134