Re: [Qemu-devel] VFIO use of HOST_PAGE_ALIGN

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 06/01/2015 04:27 AM, Peter Crosthwaite wrote:
> On Sun, May 31, 2015 at 6:34 AM, Alexey Kardashevskiy <aik@ozlabs.ru> 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 >=
>         TARGET_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.


Ah, just this one. TARGET_PAGE_SIZE is 4K for PPC64 and the actual page 
size can be 4K and 64K so that branch never works for PPC64.


>>> 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
> alignment".

Good, I like it. Cannot it be just a qemu_host_page_size? It is a bit 
confusing to have both qemu_host_page_size and qemu_real_host_page_size 
when even the first name suggests it is something non-static and sort of 
real :)



-- 
Alexey