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Date: Mon, 30 May 2016 16:14:09 +0800
From: Peter Xu <peterx@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v7 07/25] intel_iommu: define several
 structs for IOMMU IR
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To: Jan Kiszka <jan.kiszka@web.de>
Cc: David Kiarie <davidkiarie4@gmail.com>, QEMU Developers <qemu-devel@nongnu.org>, imammedo@redhat.com, rth@twiddle.net, ehabkost@redhat.com, jasowang@redhat.com, Marcel Apfelbaum <marcel@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>, pbonzini@redhat.com, rkrcmar@redhat.com, alex.williamson@redhat.com, wexu@redhat.com

On Mon, May 30, 2016 at 07:56:16AM +0200, Jan Kiszka wrote:
> On 2016-05-30 07:45, Peter Xu wrote:
> > On Sun, May 29, 2016 at 11:21:35AM +0300, David Kiarie wrote:
> > [...]
> >>>> +
> >>>> +/* Programming format for MSI/MSI-X addresses */
> >>>> +union VTD_IR_MSIAddress {
> >>>> +    struct {
> >>>> +        uint8_t __not_care:2;
> >>>> +        uint8_t index_h:1;          /* Interrupt index bit 15 */
> >>>> +        uint8_t sub_valid:1;        /* SHV: Sub-Handle Valid bit */
> >>>> +        uint8_t int_mode:1;         /* Interrupt format */
> >>>> +        uint16_t index_l:15;        /* Interrupt index bit 14-0 */
> >>>> +        uint16_t __head:12;         /* Should always be: 0x0fee */
> >>>> +    } QEMU_PACKED;
> >>>> +    uint32_t data;
> >>>> +};
> >>>
> >>> In a recent discussion, it was brought to my attention that you might
> >>> have a problem with bitfields when the host cpu is not x86. Have you
> >>> considered this ?
> >>
> >> In a case when say the host cpu is little endian.
> > 
> > I assume you mean when host cpu is big endian. x86 was little endian,
> > and I was testing on x86.
> > 
> > I think you are right. I should do conditional byte swap for all
> > uint{16/32/64} cases within the fields. For example, index_l field in
> > above VTD_IR_MSIAddress. And there are several other cases that need
> > special treatment in the patchset. Will go over and fix corresponding
> > issues in next version.
> 
> You actually need bit-swap with bit fields, see e.g. hw/net/vmxnet3.h.

Not noticed about bit-field ordering before... So maybe I need both?

Thanks,

-- peterx