linux-kernel - Re: [patch 0/6] x86/fpu: Preparatory changes for guest AMX support

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Date:   Tue, 14 Dec 2021 14:24:51 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Jing Liu <jing2.liu@...ux.intel.com>,
        Yang Zhong <yang.zhong@...el.com>, x86@...nel.org,
        kvm@...r.kernel.org, Sean Christoperson <seanjc@...gle.com>,
        Jin Nakajima <jun.nakajima@...el.com>,
        Kevin Tian <kevin.tian@...el.com>
Subject: Re: [patch 0/6] x86/fpu: Preparatory changes for guest AMX support

On Tue, Dec 14 2021 at 11:42, Paolo Bonzini wrote:
> On 12/14/21 03:50, Thomas Gleixner wrote:
>> The only remaining issue is the KVM XSTATE save/restore size checking which
>> probably requires some FPU core assistance. But that requires some more
>> thoughts vs. the IOCTL interface extension and once that is settled it
>> needs to be solved in one go. But that's an orthogonal issue to the above.
>
> That's not a big deal because KVM uses the uncompacted format.  So 
> KVM_CHECK_EXTENSION and KVM_GET_XSAVE can just use CPUID to retrieve the 
> size and uncompacted offset of the largest bit that is set in 
> kvm_supported_xcr0, while KVM_SET_XSAVE can do the same with the largest 
> bit that is set in the xstate_bv.

For simplicity you can just get that information from guest_fpu. See
below.

Thanks,

        tglx
---
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -518,6 +518,11 @@ struct fpu_guest {
 	u64				perm;
 
 	/*
+	 * @uabi_size:			Size required for save/restore
+	 */
+	unsigned int			uabi_size;
+
+	/*
 	 * @fpstate:			Pointer to the allocated guest fpstate
 	 */
 	struct fpstate			*fpstate;
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -240,6 +240,7 @@ bool fpu_alloc_guest_fpstate(struct fpu_
 	gfpu->fpstate		= fpstate;
 	gfpu->xfeatures		= fpu_user_cfg.default_features;
 	gfpu->perm		= fpu_user_cfg.default_features;
+	gfpu->uabi_size		= fpu_user_cfg.default_size;
 	fpu_init_guest_permissions(gfpu);
 
 	return true;
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1545,6 +1545,7 @@ static int fpstate_realloc(u64 xfeatures
 		newfps->is_confidential = curfps->is_confidential;
 		newfps->in_use = curfps->in_use;
 		guest_fpu->xfeatures |= xfeatures;
+		guest_fpu->uabi_size = usize;
 	}
 
 	fpregs_lock();