[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Qemu-devel] buffer alignment for block backends

From: Anthony Liguori
Subject: Re: [Qemu-devel] buffer alignment for block backends
Date: Thu, 09 Apr 2009 10:56:27 -0500
User-agent: Thunderbird (X11/20090320)

Samuel Thibault wrote:
Anthony Liguori, le Thu 09 Apr 2009 08:11:27 -0500, a écrit :
- ide.c:ide_init2
This buffer is only used when not doing DMA. When doing DMA, we are able to do zero-copy IO so the alignment of the request depends on how the guest aligned the request. I suspect you'll find a lot of guests that, in practice, do not align requests at 4k boundaries.

Eeeeerrr, why shoudn't they?  Guests usually work on pages, which are
already aligned on 4k boundaries.

Like if you do a 512-byte O_DIRECT write (in userspace) to a 512-byte, but not 4096-byte aligned buffer in Linux. This may not happen all the time, but if you don't handle this in your backend, you'll get a lot of lovely bug reports :-)


Anthony Liguori

reply via email to

[Prev in Thread] Current Thread [Next in Thread]