linux-kernel - Re: [PATCH v9 03/24] virt: sev-guest: Make payload a variable length array

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Date: Fri, 21 Jun 2024 18:54:10 +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 v9 03/24] virt: sev-guest: Make payload a variable length
 array

On Fri, May 31, 2024 at 10:00:17AM +0530, Nikunj A Dadhania wrote:
> Currently, guest message is PAGE_SIZE bytes and payload is hard-coded to
> 4000 bytes, assuming snp_guest_msg_hdr structure as 96 bytes.
> 
> Remove the structure size assumption and hard-coding of payload size and
> instead use variable length array.

I don't understand here what hard-coding is being removed?

It is simply done differently:

from

> -     snp_dev->request = alloc_shared_pages(dev, sizeof(struct snp_guest_msg));

to

> +     snp_dev->request = alloc_shared_pages(dev, SNP_GUEST_MSG_SIZE);

Maybe I'm missing the point here but do you mean by removing the hard-coding
this:

+#define SNP_GUEST_MSG_SIZE 4096
+#define SNP_GUEST_MSG_PAYLOAD_SIZE (SNP_GUEST_MSG_SIZE - sizeof(struct snp_guest_msg))

where the msg payload size will get computed at build time and you won't have
to do that 4000 in the struct definition:

	u8 payload[4000];

?

-- 
Regards/Gruss,
    Boris.

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