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

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Date:   Sat, 8 Oct 2022 15:59:41 +0300
From:   Xenia Ragiadakou <burzalodowa@...il.com>
To:     Oleksandr Tyshchenko <Oleksandr_Tyshchenko@...m.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 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.

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

-- 
Xenia