linux-kernel - Re: [PATCH v10 08/24] virt: sev-guest: Take mutex in snp_send_guest

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Date: Fri, 28 Jun 2024 10:39:33 +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 v10 08/24] virt: sev-guest: Take mutex in
 snp_send_guest_request()

On Fri, Jun 21, 2024 at 06:08:47PM +0530, Nikunj A Dadhania wrote:
> SNP command mutex is used to serialize access to the shared buffer, command
> handling and message sequence number races.

serialize access to ... races?

Needs re-formulation.

> As part of the preparation for moving SEV guest driver common code and
> making mutex private, take the mutex in snp_send_guest_request() instead of
> snp_guest_ioctl(). This will result in locking behavior change as detailed
> below:
> 
> Current locking behaviour:
> 
>     snp_guest_ioctl()
>       mutex_lock(&snp_cmd_mutex)
>         get_report()/get_derived_key()/get_ext_report()
>           snp_send_guest_request()
>     	...
>       mutex_unlock(&snp_cmd_mutex)
> 
> New locking behaviour:
> 
>     snp_guest_ioctl()
>       get_report()/get_derived_key()/get_ext_report()
>         snp_send_guest_request()
>            guard(mutex)(&snp_cmd_mutex)
>              ...

Why is it ok to grab the mutex in snp_send_guest_request()?

Folks need to learn to stop spelling out what the patch does but WHY it
does it and WHY is it ok?!?

> Remove multiple lockdep check in the sev-guest driver as they are redundant
> now.

More "what" redundancy.

"The new locking region covers <bla> and that is ok because of <foo>."

This is what your commit message should say.

-- 
Regards/Gruss,
    Boris.

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