Re: [Qemu-devel] [PATCH v7 07/25] intel_iommu: define several structs for IOMMU IR

[Jan Kiszka <jan.kiszka@web.de>]

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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.

Jan



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