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

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Date: Sun, 23 Jun 2024 21:46:09 +0530
From: "Nikunj A. Dadhania" <nikunj@....com>
To: Borislav Petkov <bp@...en8.de>
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 6/21/2024 10:24 PM, Borislav Petkov wrote:
> 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];
> 
> ?

Yes, payload was earlier fixed at 4000 bytes, without considering the size of snp_guest_msg.

Regards
Nikunj