linux-kernel - Re: [PATCH] KVM: VMX: Do not trap VMFUNC instructions for L1 guests.

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Date:   Tue, 8 Nov 2022 18:41:52 +0800
From:   Yu Zhang <yu.c.zhang@...ux.intel.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     seanjc@...gle.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: VMX: Do not trap VMFUNC instructions for L1 guests.

On Mon, Nov 07, 2022 at 06:20:23PM +0100, Paolo Bonzini wrote:
> On 11/7/22 09:27, Yu Zhang wrote:
> > VMFUNC is not supported for L1 guests, and executing VMFUNC in
> > L1 shall generate a #UD directly. Just disable it in secondary
> > proc-based execution control for L1, instead of intercepting it
> > and inject the #UD again.
> > 
> > Signed-off-by: Yu Zhang<yu.c.zhang@...ux.intel.com>
> 
> Is this for TDX or similar?  The reason for a patch should be mentioned in
> the commit message.

Thanks for your quick reply, Paolo. 

It is not a new feature. Just a clean up for VMFUNC, which is not
supported by KVM for L1 guest.

According to Intel SDM 25.5.6.2 - "General Operation of the VMFUNC
Instruction", The VMFUNC instruction causes an invalid-opcode exception
(#UD) if the “enable VM functions” VM-execution controls is 0 or the
value of EAX is greater than 63 (only VM functions 0–63 can be enable).
Otherwise, the instruction causes a VM exit if the bit at position
EAX is 0 in the VM-function controls (the selected VM function is not
enabled)

And since KVM only provides emulation of VMFUNC for nested guests,
it is uncessary for KVM to intercept it and reinject a #UD. So just
disable VMFUNC in VM-execution control for L1 guests.

But please feel free to educate me if I missed some backgrounds about
why this is enabled in the first place. Thanks!

B.R.
Yu