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Re: [Qemu-devel] VFIO use of HOST_PAGE_ALIGN

From: Peter Crosthwaite
Subject: Re: [Qemu-devel] VFIO use of HOST_PAGE_ALIGN
Date: Sun, 31 May 2015 11:27:19 -0700

On Sun, May 31, 2015 at 6:34 AM, Alexey Kardashevskiy <address@hidden> wrote:
> 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?

In translate-all.c:

void page_size_init(void)
    /* NOTE: we can always suppose that qemu_host_page_size >=
    qemu_real_host_page_size = getpagesize();
    if (qemu_host_page_size == 0) {
        qemu_host_page_size = qemu_real_host_page_size;
    if (qemu_host_page_size < TARGET_PAGE_SIZE) {
        qemu_host_page_size = TARGET_PAGE_SIZE;
    qemu_host_page_mask = ~(qemu_host_page_size - 1);

It is clamped to be at least as big as a the 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" :)

So the change would be an alternate macro based on
qemu_real_host_page_size which is what I am calling "raw host page


> --
> Alexey

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