linux-kernel - Re: [PATCH] xen/virtio: Handle cases when page offset > PAGE

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Date:   Sat, 8 Oct 2022 13:21:44 +0000
From:   Oleksandr Tyshchenko <Oleksandr_Tyshchenko@...m.com>
To:     Xenia Ragiadakou <burzalodowa@...il.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Stefano Stabellini <sstabellini@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Oleksandr Tyshchenko <olekstysh@...il.com>
Subject: Re: [PATCH] xen/virtio: Handle cases when page offset > PAGE_SIZE
 properly


On 08.10.22 15:59, Xenia Ragiadakou wrote:

Hello Xenia

>
> On 10/8/22 15:52, Oleksandr Tyshchenko wrote:
>>
>> On 08.10.22 14:08, Xenia Ragiadakou wrote:
>>
>> Hello Xenia
>>
>>>
>>> On 10/7/22 16:27, Oleksandr Tyshchenko wrote:
>>>
>>> Hi Oleksandr
>>>
>>>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
>>>>
>>>> Passed to xen_grant_dma_map_page() offset in the page
>>>> can be > PAGE_SIZE even if the guest uses the same page granularity
>>>> as Xen (4KB).
>>>>
>>>> Before current patch, if such case happened we ended up providing
>>>> grants for the whole region in xen_grant_dma_map_page() which
>>>> was really unnecessary. The more, we ended up not releasing all
>>>> grants which represented that region in xen_grant_dma_unmap_page().
>>>>
>>>> Current patch updates the code to be able to deal with such cases.
>>>>
>>>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
>>>> ---
>>>> Cc: Juergen Gross <jgross@...e.com>
>>>> Cc: Xenia Ragiadakou <burzalodowa@...il.com>
>>>>
>>>> Depens on:
>>>> https://urldefense.com/v3/__https://lore.kernel.org/xen-devel/20221005174823.1800761-1-olekstysh@gmail.com/__;!!GF_29dbcQIUBPA!xnkNaKpfZ4LssQJcJs_J91KERZKMP2Rd-xEdBqXNXJ8GyCXJ0gkRer1elVYfxOWtwN_FOl9tVieDWlfN-UZaHQsyLMhA$ 
>>>>
>>>> [lore[.]kernel[.]org]
>>>>
>>>> Should go in only after that series.
>>>> ---
>>>>    drivers/xen/grant-dma-ops.c | 8 +++++---
>>>>    1 file changed, 5 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
>>>> index c66f56d24013..1385f0e686fe 100644
>>>> --- a/drivers/xen/grant-dma-ops.c
>>>> +++ b/drivers/xen/grant-dma-ops.c
>>>> @@ -168,7 +168,9 @@ static dma_addr_t xen_grant_dma_map_page(struct
>>>> device *dev, struct page *page,
>>>>                         unsigned long attrs)
>>>>    {
>>>>        struct xen_grant_dma_data *data;
>>>> -    unsigned int i, n_pages = PFN_UP(offset + size);
>>>> +    unsigned long dma_offset = offset_in_page(offset),
>>>> +            gfn_offset = PFN_DOWN(offset);
>>>> +    unsigned int i, n_pages = PFN_UP(dma_offset + size);
>>>
>>> IIUC, the above with a later patch will become:
>>>
>>> dma_offset = xen_offset_in_page(offset)
>>> gfn_offset = XEN_PFN_DOWN(offset)
>>> n_pages = XEN_PFN_UP(dma_offset + size)
>>
>>
>> If saying "later" patch you meant "xen/virtio: Convert
>> PAGE_SIZE/PAGE_SHIFT/PFN_UP to Xen counterparts" then yes, exactly.
>
> Ah ok, I see.
>
>>>
>>>
>>>>        grant_ref_t grant;
>>>>        dma_addr_t dma_handle;
>>>>    @@ -187,10 +189,10 @@ static dma_addr_t
>>>> xen_grant_dma_map_page(struct device *dev, struct page *page,
>>>>          for (i = 0; i < n_pages; i++) {
>>>>            gnttab_grant_foreign_access_ref(grant + i,
>>>> data->backend_domid,
>>>> -                xen_page_to_gfn(page) + i, dir == DMA_TO_DEVICE);
>>>> +                xen_page_to_gfn(page) + i + gfn_offset, dir ==
>>>> DMA_TO_DEVICE);
>>>
>>> Here, why the pfn is not calculated before passing it to pfn_to_gfn()?
>>> I mean sth like pfn_to_gfn(page_to_xen_pfn(page) + gfn_offset + i)
>>
>> The gfn_offset is just a const value here, which just means how many
>> gfns we should skip. But ...
>>
>> ... I think, I get your point. So, if the region which is contiguous in
>> pfn might be non-contiguous in gfn (which seems to be the case for x86's
>> PV, but I may mistake) we should indeed use open-coded
>>
>> construction "pfn_to_gfn(page_to_xen_pfn(page) + gfn_offset + i)".  And
>> the gfn_offset should be renamed to pfn_offset then.
>>
>>
>> Correct?
>
> Yes, that 's what I had in mind unless I 'm missing sth.


ok, thanks for confirming. So I will create V2 then.


>
>>>
>>>>        }
>>>>    -    dma_handle = grant_to_dma(grant) + offset;
>>>> +    dma_handle = grant_to_dma(grant) + dma_offset;
>>>>          return dma_handle;
>>>>    }
>>>
>
-- 
Regards,

Oleksandr Tyshchenko