linux-kernel - Re: [PATCH v6 02/25] x86/fpu/xstate: Fix guest fpstate allocation size calculation

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Message-ID: <ZTMdyR8e63sCTKWc@google.com>
Date:   Fri, 20 Oct 2023 17:39:37 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     pbonzini@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, dave.hansen@...el.com,
        peterz@...radead.org, chao.gao@...el.com,
        rick.p.edgecombe@...el.com, john.allen@....com
Subject: Re: [PATCH v6 02/25] x86/fpu/xstate: Fix guest fpstate allocation
 size calculation

On Thu, Sep 14, 2023, Yang Weijiang wrote:
> Fix guest xsave area allocation size from fpu_user_cfg.default_size to
> fpu_kernel_cfg.default_size so that the xsave area size is consistent
> with fpstate->size set in __fpstate_reset().
> 
> With the fix, guest fpstate size is sufficient for KVM supported guest
> xfeatures.
> 
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> ---
>  arch/x86/kernel/fpu/core.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index a86d37052a64..a42d8ad26ce6 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -220,7 +220,9 @@ bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
>  	struct fpstate *fpstate;
>  	unsigned int size;
>  
> -	size = fpu_user_cfg.default_size + ALIGN(offsetof(struct fpstate, regs), 64);
> +	size = fpu_kernel_cfg.default_size +
> +	       ALIGN(offsetof(struct fpstate, regs), 64);

Shouldn't all the other calculations in this function also switch to fpu_kernel_cfg?
At the very least, this looks wrong when paired with the above:

	gfpu->uabi_size		= sizeof(struct kvm_xsave);
	if (WARN_ON_ONCE(fpu_user_cfg.default_size > gfpu->uabi_size))
		gfpu->uabi_size = fpu_user_cfg.default_size;