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Re: [Xen-devel] Re: [PATCH] Linux: 6 arguments hypercall v3

To: James <James.McKenzie@xxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH] Linux: 6 arguments hypercall v3
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Sun, 03 Jul 2011 00:05:02 +0100
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir@xxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, Jean Guyader <jean.guyader@xxxxxxxxxxxxx>, Jean Guyader <Jean.Guyader@xxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>
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On 06/27/2011 09:19 PM, James wrote:
> On Fri, Jun 24, 2011 at 03:43:18PM -0700, Jeremy Fitzhardinge wrote:
>> On 06/24/2011 03:15 PM, Jean Guyader wrote:
>>> I only tested it gcc 4.4.
>>> We could still use the v2 which is not as nice, but it doesn't try to
>>> load eax twice. Let me know which one you will feel better about.
> Given "that loading r0 twice" is in the gcc documentation as an
> example at
> http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#index-extended-_0040code_007basm_007d-2601
> I think it's fair to use that.
>> I feel best about the one you've successfully tested with a number of
>> older compilers ;)
> what compilers work to build modern linux? We'll give a goodly set of them
> a try if that makes everyone feel better.

HPA says:
> For x86, we support 3.4, 4.0 and 4.1.2 and above (not sure if 4.0.x
> actually works).

>> Does it have to?  Couldn't it take a pointer to a struct or something?
> Yes we could change the API, make sure the struct is visible on all
> CPUs, making it incompatible with the existing code, but I think that
> misses the point: Xen is written to support 6 argument hypercalls, but
> 32 bit dom0 is missing the functionality to call them. Surely that's 
> something we should fix?

The nominal ability of 6 arg hypercalls is moot if there aren't actually
any hypercalls which have 6 args.  The fact that 6 args uses ebp on
32-bit is incredibly awkward, and it seems better to just say that it in
fact only supports 5, and adjust the v4v call(s) accordingly.


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