linux-kernel - Re: [PATCH 1/1] scsi: storvsc: Don't pass unused PFNs to Hyper-V host

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Date:   Tue, 16 May 2023 22:13:26 -0400
From:   "Martin K. Petersen" <martin.petersen@...cle.com>
To:     kys@...rosoft.com, longli@...rosoft.com, wei.liu@...nel.org,
        decui@...rosoft.com, jejb@...ux.ibm.com,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-scsi@...r.kernel.org, Michael Kelley <mikelley@...rosoft.com>
Cc:     "Martin K . Petersen" <martin.petersen@...cle.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/1] scsi: storvsc: Don't pass unused PFNs to Hyper-V host

On Mon, 15 May 2023 10:20:41 -0700, Michael Kelley wrote:

> In a SCSI request, storvsc pre-allocates space for up to
> MAX_PAGE_BUFFER_COUNT physical frame numbers to be passed to Hyper-V.
> If the size of the I/O request requires more PFNs, a separate memory
> area of exactly the correct size is dynamically allocated.
> 
> But when the pre-allocated area is used, current code always passes
> MAX_PAGE_BUFFER_COUNT PFNs to Hyper-V, even if fewer are needed.  While
> this doesn't break anything because the additional PFNs are always zero,
> more bytes than necessary are copied into the VMBus channel ring buffer.
> This takes CPU cycles and wastes space in the ring buffer. For a typical
> 4 Kbyte I/O that requires only a single PFN, 248 unnecessary bytes are
> copied.
> 
> [...]

Applied to 6.4/scsi-fixes, thanks!

[1/1] scsi: storvsc: Don't pass unused PFNs to Hyper-V host
      https://git.kernel.org/mkp/scsi/c/4e81a6cba517

-- 
Martin K. Petersen	Oracle Linux Engineering