linux-kernel - Re: [RFC 05/33] KVM: x86: hyper-v: Introduce VTL call/return prologues in hypercall page

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Message-ID: <CWTHOA1Z96FB.16IGGD2MN12OS@amazon.com>
Date:   Wed, 8 Nov 2023 14:10:16 +0000
From:   Nicolas Saenz Julienne <nsaenz@...zon.com>
To:     Alexander Graf <graf@...zon.com>, <kvm@...r.kernel.org>
CC:     <linux-kernel@...r.kernel.org>, <linux-hyperv@...r.kernel.org>,
        <pbonzini@...hat.com>, <seanjc@...gle.com>, <vkuznets@...hat.com>,
        <anelkz@...zon.com>, <dwmw@...zon.co.uk>, <jgowans@...zon.com>,
        <corbert@....net>, <kys@...rosoft.com>, <haiyangz@...rosoft.com>,
        <decui@...rosoft.com>, <x86@...nel.org>,
        <linux-doc@...r.kernel.org>
Subject: Re: [RFC 05/33] KVM: x86: hyper-v: Introduce VTL call/return
 prologues in hypercall page

On Wed Nov 8, 2023 at 11:53 AM UTC, Alexander Graf wrote:

[...]

> > @@ -285,6 +286,81 @@ static int patch_hypercall_page(struct kvm_vcpu *vcpu, u64 data)
> >   	/* ret */
> >   	((unsigned char *)instructions)[i++] = 0xc3;
> >   
> > +	/* VTL call/return entries */
> > +	if (!kvm_xen_hypercall_enabled(kvm) && kvm_hv_vsm_enabled(kvm)) {
>
>
> You don't introduce kvm_hv_vsm_enabled() before. Please do a quick test 
> build of all individual commits of your patch set for v1 :).

Yes, sorry for that. This happens for a couple of helpers, I'll fix it.

> Why do you need the ifdef here? is_long_mode() already has an ifdef that 
> will always return false for is_64_bit_mode() on 32bit hosts.

Noted, will remove.

> > +		if (is_64_bit_mode(vcpu)) {
> > +			/*
> > +			 * VTL call 64-bit entry prologue:
> > +			 * 	mov %rcx, %rax
> > +			 * 	mov $0x11, %ecx
> > +			 * 	jmp 0:
> > +			 */
> > +			hv->vsm_code_page_offsets.vtl_call_offset = i;
> > +			instructions[i++] = 0x48;
> > +			instructions[i++] = 0x89;
> > +			instructions[i++] = 0xc8;
> > +			instructions[i++] = 0xb9;
> > +			instructions[i++] = 0x11;
> > +			instructions[i++] = 0x00;
> > +			instructions[i++] = 0x00;
> > +			instructions[i++] = 0x00;
> > +			instructions[i++] = 0xeb;
> > +			instructions[i++] = 0xe0;
>
>
> I think it would be a lot easier to read (because it's denser) if you 
> move the opcodes into a character array:
>
> char vtl_entry[] = { 0x48, 0x89, 0xc8, 0xb9, 0x11, 0x00, 0x00, 0x00. 
> 0xeb, 0xe0 };
>
> and then just memcpy().

Works for me, I'll rework it.

Nicolas