linux-kernel - Re: [PATCH v8 04/16] virt: sev-guest: Add vmpck_id to snp_guest

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Date: Tue, 16 Apr 2024 11:06:58 +0200
From: Borislav Petkov <bp@...en8.de>
To: "Nikunj A. Dadhania" <nikunj@....com>
Cc: linux-kernel@...r.kernel.org, thomas.lendacky@....com, x86@...nel.org,
	kvm@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de,
	dave.hansen@...ux.intel.com, pgonda@...gle.com, seanjc@...gle.com,
	pbonzini@...hat.com
Subject: Re: [PATCH v8 04/16] virt: sev-guest: Add vmpck_id to snp_guest_dev
 struct

On Tue, Apr 16, 2024 at 11:27:24AM +0530, Nikunj A. Dadhania wrote:
> > Why does that snp_get_os_area_msg_seqno() returns a pointer when you
> > deref it here again?
> > 
> > A function which returns a sequence number should return that number
> > - not a pointer to it.
> > 
> > Which then makes that u32 *os_area_msg_seqno redundant and you can use
> > the function directly.
> > 
> > IOW:
> > 
> > static inline u32 snp_get_os_area_msg_seqno(struct snp_guest_dev *snp_dev)
> > {
> >         return snp_dev->layout->os_area.msg_seqno_0 + snp_dev->vmpck_id;
> 
> This patch removes setting of layour page in snp_dev structure.

So?

> static inline u32 snp_get_os_area_msg_seqno(struct snp_guest_dev *snp_dev)
> {
>         if (!platform_data)
>                 return NULL;
> 
>         return *(&platform_data->layout->os_area.msg_seqno_0 + vmpck_id);
> }

What?!

This snp_get_os_area_msg_seqno() is a new function added by this patch.

> I had a getter for getting the os_area_msg_seqno pointer, probably not
> a good function name.

Probably you need to go back to the drawing board and think about how
this thing should look like.

> > Do you see the imbalance in the APIs?
> 
> The msg_seqno should only be incremented by 2 (always), that was the reason to avoid a setter.

And what's wrong with the setter doing the incrementation so that
callers can't even get it wrong?

It sounds to me like you should redesign this sequence number handling
in a *separate* patch.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette