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Re: [Qemu-devel] VFIO use of HOST_PAGE_ALIGN

From: Alexey Kardashevskiy
Subject: Re: [Qemu-devel] VFIO use of HOST_PAGE_ALIGN
Date: Sun, 31 May 2015 23:34:01 +1000
User-agent: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 05/27/2015 01:22 AM, Alex Williamson wrote:
[cc +alexey]

On Mon, 2015-05-25 at 00:48 -0700, Peter Crosthwaite wrote:
Hi Alex and all,

I am working on a patch series to enable multiple CPU architectures to
run at once. It's a long story, but I have hit a snag in hw/vfio/pci.c
which AFAICS is the only in-tree system-mode use of HOST_PAGE_ALIGN
(all usermode code is a non-issue as not looking to support user-mode
multi-arch yet).

The problem I face, is that this macro depends on translate-all.c's
qemu_host_page_size which in turn, depends on TARGET_PAGE_SIZE. I'm
hoping that one day, TARGET_PAGE_SIZE will be a variable and the users
of it will know to get the correct value depending on their CPU
specific code location. vfio is the only one I can't handle. My
knowledge on vfio is near-0, but my thinking is, since this is not
arch specific code can we instead use the raw host page alignment
rather that the CPU arch specific one?

What is "raw host page" here? I thought qemu_host_page_size is the one, where does it depend on TARGET_PAGE_SIZE?

I think we could replace our use of HOST_PAGE_ALIGN with something based
only on the host's getpagesize().  I don't see that we really care about
the target page size for this usage.  Alexey, I think you're the only
arch where host and target page sizes can actually be different, do you
agree?  Thanks,

Strongly agree. Where it really matters (MSIX), it is already qemu_host_page_size and HOST_PAGE_ALIGN and I am a bit scared by that "raw host page alignment" :)


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