linux-kernel - Re: [PATCH v8 04/12] tpm: Change tpm_get

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Message-ID: <aVf0V7GOtekOiSi7@kernel.org>
Date: Fri, 2 Jan 2026 18:37:43 +0200
From: Jarkko Sakkinen <jarkko@...nel.org>
To: Jonathan McDowell <noodles@...th.li>
Cc: linux-integrity@...r.kernel.org,
	"David S . Miller" <davem@...emloft.net>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Eric Biggers <ebiggers@...nel.org>, Peter Huewe <peterhuewe@....de>,
	Jason Gunthorpe <jgg@...pe.ca>, David Howells <dhowells@...hat.com>,
	Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:KEYS/KEYRINGS" <keyrings@...r.kernel.org>,
	"open list:SECURITY SUBSYSTEM" <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v8 04/12] tpm: Change tpm_get_random() opportunistic

On Fri, Dec 19, 2025 at 09:42:07AM +0000, Jonathan McDowell wrote:
> On Tue, Dec 16, 2025 at 11:21:38AM +0200, Jarkko Sakkinen wrote:
> > hwrng framework does not have a requirement that the all bytes requested
> > need to be provided. By enforcing such a requirement internally, TPM driver
> > can cause unpredictability in latency, as a single tpm_get_random() call
> > can result multiple TPM commands.
> > 
> > Especially, when TCG_TPM2_HMAC is enabled, extra roundtrips could have
> > significant effect to the system latency.
> > 
> > Thus, send TPM command only once and return bytes received instead of
> > committing to the number of requested bytes.
> 
> Function comment for tpm_get_random needs updated as well, as it currently
> says "until all of the @max bytes have been received", which is no longer
> true with this patch. With that:
> 
> Reviewed-by: Jonathan McDowell <noodles@...a.com>

Thank you and definitely can refine that comment. After holidays it is
probably to go through this patch set with time and send +1 iteration
:-)

BR, Jarkko