linux-kernel - Re: [PATCH 09/19] kvm: x86: Prepare reallocation check

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Date:   Mon, 13 Dec 2021 10:16:27 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yang Zhong <yang.zhong@...el.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com
Cc:     seanjc@...gle.com, jun.nakajima@...el.com, kevin.tian@...el.com,
        jing2.liu@...ux.intel.com, jing2.liu@...el.com
Subject: Re: [PATCH 09/19] kvm: x86: Prepare reallocation check

On 12/8/21 01:03, Yang Zhong wrote:
> +	u64 xcr0 = vcpu->arch.xcr0 & XFEATURE_MASK_USER_DYNAMIC;
> +
> +	/* For any state which is enabled dynamically */
> +	if ((xfd & xcr0) != xcr0) {
> +		u64 request = (xcr0 ^ xfd) & xcr0;
> +		struct fpu_guest *guest_fpu = &vcpu->arch.guest_fpu;
> +
> +		/*
> +		 * If requested features haven't been enabled, update
> +		 * the request bitmap and tell the caller to request
> +		 * dynamic buffer reallocation.
> +		 */
> +		if ((guest_fpu->user_xfeatures & request) != request) {
> +			vcpu->arch.guest_fpu.realloc_request = request;

This should be "|=".  If you have

	wrmsr(XFD, dynamic-feature-1);
	...
	wrmsr(XFD, dynamic-feature-2);

then the space for both features has to be allocated.

> +			return true;
> +		}
> +	}
> +


This is just:

	struct fpu_guest *guest_fpu = &vcpu->arch.guest_fpu;
	u64 xcr0 = vcpu->arch.xcr0 & XFEATURE_MASK_USER_DYNAMIC;
	u64 dynamic_enabled = xcr0 & ~xfd;
	if (!(dynamic_enabled & ~guest_fpu->user_xfeatures))
		return false;

	/*
	 * This should actually not be in guest_fpu, see review of
	 * patch 2.  Also see above regarding "=" vs "|=".
	 */
	vcpu->arch.guest_fpu.realloc_request |= dynamic_enabled;
	return true;

But without documentation I'm not sure why this exit-to-userspace step 
is needed:

- if (dynamic_enabled & ~guest_fpu->user_perm) != 0, then this is a 
userspace error and you can #GP the guest without any issue.  Userspace 
is buggy

- if (dynamic_enabled & ~guest_fpu->user_xfeatures) != 0, but the 
feature *is* permitted, then it is okay to just go on and reallocate the 
guest FPU.


Paolo