Re: [PATCH v4 2/2] x86/ioreq: Extend ioreq server to support multiple ioreq pages

["Julian Vetter" <julian.vetter@xxxxxxxxxx>]

On 3/10/26 17:09, Jan Beulich wrote:
> On 10.03.2026 16:56, Julian Vetter wrote:
>> On 3/10/26 16:36, Jan Beulich wrote:
>>> On 05.03.2026 14:04, Julian Vetter wrote:
>>>> @@ -45,7 +52,8 @@ struct ioreq_server {
>>>>        /* Lock to serialize toolstack modifications */
>>>>        spinlock_t             lock;
>>>>    
>>>> -    struct ioreq_page      ioreq;
>>>> +    ioreq_t                *ioreq;
>>>> +    gfn_t                  ioreq_gfn;
>>>>        struct list_head       ioreq_vcpu_list;
>>>>        struct ioreq_page      bufioreq;
>>>
>>> This change in data arrangement should in principle be independent of the
>>> step to supporting multiple pages. Hence it should be possible to separate
>>> out. Problem being that just by looking here and at hvm_{,un}map_ioreq_gfn()
>>> I can't conclude how you get away without the "page" field that struct
>>> ioreq_page had. If you can get away without, it's not quite clear why the
>>> field exists in the first place. If it's not needed, dropping it would be
>>> yet another separate, prereq change. At which point the remaining pair of
>>> fields could continue to be used, i.e. the change above then wouldn't be
>>> needed; va could be renamed if need be, and its type changed.
>>
>> Thank you again Jan for your feedback! I don't need the page anymore.
>> When I use vmap(), I don't need to keep track of it, because during
>> teardown, I can recover it via vmap_to_page(). Currently it's necessary
>> because in destroy_ring_for_helper we need the page, to be destroyed.
>> But I see now, on X86 the map_domain_page_global called from
>> prepare_ring_for_helper actually does vmap(&mfn, 1). So the page is also
>> from the vmap range. So for the teardown I assume a vmap_to_page() could
>> be used as well. But I also see there is a special case, if NDEBUG=1,
>> map_domain_page_global short-circuits to mfn_to_virt() for low MFNs
>> (putting the VA in the directmap range) and bypassing vmap. In that case
>>
>> vmap_to_page() would not work. So, this would be really messy. I would
>> rather switch the bufioreq also to an explicitly vmap()'ed page, then we
>> could remove the page pointer and both cases would be aligned again.
> 
> That's an option. Yet are you aware of domain_page_map_to_mfn()? Perhaps
> that's what you want to switch to using in the patch removing the "page"
> field. To then, conditionally or uniformly, switch to vmap_to_{mfn,page}()
> when doing the multi-page work in the subsequent patch.

Yes, thank you. I saw this function, but I was wondering whether it's a 
good idea to wrap the va in two translation functions like:

struct page_info *page = mfn_to_page(domain_page_map_to_mfn(va));

and then calling destroy_ring_for_helper() with it. But I will have a 
look, and this way we would be again aligned between the two cases. So, 
maybe it's the cleanest way.

Julian

> 
> Jan



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Julian Vetter | Vates Hypervisor & Kernel Developer

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