RE: [Xen-devel] [PATCH: IOEMU & XEN] Change ioreq layout and reduce its size

["Xu, Dongxiao" <dongxiao.xu@xxxxxxxxx>]

Keir Fraser wrote:
> [Ian, please apply the attached ioemu patch and then send me an
> updated QEMU_TAG. The change to the ioreq layout requires the ioemu
> and xen-unstable patch check-ins to be closely synchronised.]
> 
> Reduce size of Xen-qemu shared ioreq structure to 32 bytes. This has
> two advantages:

Hi, Keir, 
        Reduce the ioreq structure has these advantages indeed, but do we need 
to consider the following compatibility issues? Thanks!
1) HVM guest could migrate/save/restore between the new and old hypervisor if 
vcpu number is less than 32. 
2) Old qemu could work with new hypervisor if guest vcpu number is less than 
32. 

Best Regards, 
-- Dongxiao

>  1. We can support up to 128 VCPUs with a single shared page
>  2. If/when we want to go beyond 128 VCPUs, a whole number of ioreq_t
> structures will pack into a single shared page, so a multi-page array
> will have no ioreq_t straddling a page boundary
> 
> Also, while modifying qemu, replace a 32-entry vcpu-indexed array
> with a dynamically-allocated array.
> 
> Signed-off-by: Keir Fraser <keir.fraser@xxxxxxxxxx>
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