Re: [PATCH v8 13/14] tools/libxl: explicitly grant access to needed I/O-memory ranges

[Julien Grall <julien.grall@linaro.org>]

On 26/05/14 11:11, Jan Beulich wrote:
>>>> On 25.05.14 at 19:08, <julien.grall@linaro.org> wrote:
>> On 25/05/14 11:51, Arianna Avanzini wrote:
>>> +
>>> +        /*
>>> +         * If VGA passthru is enabled by domain config, be sure that the
>>> +         * domain can access VGA-related iomem regions.
>>> +         */
>>> +        if (d_config->b_info.u.hvm.gfx_passthru.val) {
>>> +            uint64_t vga_iomem_start = 0xa0000 >> XC_PAGE_SHIFT;
>>> +            ret = xc_domain_iomem_permission(CTX->xch, domid,
>>> +                                             vga_iomem_start, 0x20, 1);
>>> +            if (ret < 0) {
>>> +                LOGE(ERROR,
>>> +                     "failed to give dom%d access to iomem range "
>>> +                     "%"PRIx64"-%"PRIx64" for VGA passthru",
>>> +                     domid, vga_iomem_start, (vga_iomem_start + 0x20 - 1));
>>> +                goto error_out;
>>> +            }
>>> +        }
>>
>> IHMO, the guest doesn't need to have permission to this region. When
>> QEMU ask to map this region to the guest, the hypercall will only check
>> the permission on the domain where QEMU is running. Therefore, the
>> permission should be given to the stubdomain.
>
> How would qemu be involved in I/O from/to a passed through
> device?

AFAIU, the mapping of the range 0xa0000-* will be done by QEMU for an 
HVM guest (i.e calling xc_domain_memory_mapping).

-- 
Julien Grall