Re: [Qemu-devel] Re: [PATCHv2] qemu: target library, use it in msix

[Blue Swirl <blauwirbel@gmail.com>]

On Tue, Sep 29, 2009 at 5:50 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Sun, Sep 27, 2009 at 06:19:05PM +0300, Blue Swirl wrote:
>> On Sun, Sep 27, 2009 at 5:24 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
>> > On Sun, Sep 27, 2009 at 04:21:29PM +0200, Michael S. Tsirkin wrote:
>> >> On Sun, Sep 27, 2009 at 04:14:49PM +0200, Avi Kivity wrote:
>> >> > On 09/27/2009 04:08 PM, Michael S. Tsirkin wrote:
>> >> >>
>> >> >>
>> >> >>>> In practice, the only user is now msix and it does not.  It has 0x1000
>> >> >>>> as a constant parameter.  For target_phys_addr_t users if we ever have
>> >> >>>> them, we'll just add target_phys_page_align. Generally it's unusual for
>> >> >>>> devices to care about size of target physical page.
>> >> >>>>
>> >> >>>>
>> >> >>> I'd fill better with uint64_t, at least that won't truncate.
>> >> >>>
>> >> >> Doesn't naming it target_page_align32 address this concern?
>> >> >>
>> >> >
>> >> > How can the caller (except in your special case) know if it has a
>> >> > quantity that will fit in 32 bits?
>> >>
>> >> It's actually not unusual for devices to limit addressing to 32 bit, whatever
>> >> the bus supports.
>> >
>> > I would say that devices normally have a specific addressing, and should
>> > not be using target specific types at all.  This alignment to target
>> > page size is actually an unusual thing.
>>
>> Actually, AFAICT MSI-X spec (6.8.2, from the MSI entry in Wikipedia)
>> only requires a QWORD alignment. There is some blurb about 4k
>> alignment, but I think it only describes how software should use the
>> structure.
>> If this is the case, we could drop the whole target page
>> stuff.
>
> The variable MSIX_PAGE_SIZE actually specifies the size of the space
> allocated for MSIX in the memory region.  Spec requires locating MSI-X
> tables in a 4K region separate from any other device register, so from
> that point of view we could just have had
> #define MSIX_PAGE_SIZE 0x1000

Can you cite the spec, I only found the QWORD stuff.

> The main reason I round the space for MSI-X tables up to cpu page size,
> is because I have to call cpu_register_physical_memory on it, which
> requires that size is a multiple of target page size.

There is no such requirement anymore (since r2868 in 2007), devices
can register regions of arbitrary size and multiple devices can share
a page.