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[Xen-devel] Re: [Qemu-devel] [PATCH 5/7] xen: add block device backend d

To: Blue Swirl <blauwirbel@xxxxxxxxx>
Subject: [Xen-devel] Re: [Qemu-devel] [PATCH 5/7] xen: add block device backend driver.
From: Gerd Hoffmann <kraxel@xxxxxxxxxx>
Date: Mon, 04 Aug 2008 21:50:50 +0200
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Blue Swirl wrote:
> On 8/4/08, Gerd Hoffmann <kraxel@xxxxxxxxxx> wrote:
>>  +/* i386 protocol version */
>>  +#pragma pack(push, 4)
> What's wrong with __attribute__(__aligned__)?

That one is tricky because we must be binary compatible with xen.  And
the block driver protocol has a small design flaw:  It has a 64bit value
 which is not aligned on a 64bit border.  This leads to different struct
layouts on i386 and x86_64 due to different alignment rules.  Unless you
force them into something else, like we do in that header file to deal
with it.  Which is needed to run 32bit guests on 64bit hosts.

>>  +    RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(&blkdev->rings.common, 
>> send_notify);

Xen API, no way around that, sorry.

>>  +       mode   = O_RDONLY;
>>  +       qflags = BDRV_O_RDONLY;
> Does this mean that all read-only disks are assumed to be CDROMs or
> that all removable disks are read-only?

I'll double-check, but I think there is no media=[disk|cdrom] indicator
in xenstore.  So that was the best I could come up with ...

> Why IF_XEN?

I've seen you've noticed meanwhile in patch #7 ;)

> Why different protocols for i386 and x86_64?

See above.

> Would I need to add
> Sparc32 and Sparc64 versions some day?

Not sure whenever the same 32/64bit ABI issue exists on sparc.  If so,
the code handling this on x86 should work for sparc too.



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