Re: [PATCH 2/2] hw/usb/hcd-xhci.c: allow unaligned access to Capability Registers

[Peter Xu]

On Mon, Dec 11, 2023 at 01:57:42PM +0000, Peter Maydell wrote:
> On Mon, 11 Dec 2023 at 07:13, Tomoyuki HIROSE
> <tomoyuki.hirose@igel.co.jp> wrote:
> >
> > According to xHCI spec rev 1.2, unaligned access to xHCI Host
> > Controller Capability Registers is not prohibited. In Addition, the
> > limit of access size is also unspecified. Actually, some real devices
> > allow unaligned access and 8-byte access to these registers.
> > This commit makes it possible to unaligned access and 8-byte access
> > to Host Controller Capability Registers.
> 
> We should definitely look at fixing the unaligned access
> stuff, but the linked bug report is not trying to do an
> unaligned access -- it wants to do a 2-byte read from offset 2,
> which is aligned.

IIUC the issue is the xHCI xhci_cap_ops defines impl.min_access_size=4.
Then it'll be enlarged to 4 bytes read on offset 0x2.  IOW, I think it'll
also work if xhci_cap_ops can support impl.min_access_size=2, however I
don't know whether that can be more than that register, so that the memory
patch is still a more generic approach.

Thanks,

-- 
Peter Xu