Re: [PATCH] xen/virtio: Convert PAGE_SIZE/PAGE_SHIFT/PFN_UP to Xen counterparts

[Oleksandr Tyshchenko <Oleksandr_Tyshchenko@xxxxxxxx>]

On 06.10.22 20:59, Xenia Ragiadakou wrote:

Hello Xenia

>
> On 10/6/22 15:09, Oleksandr Tyshchenko wrote:
>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>>
>> Although XEN_PAGE_SIZE is equal to PAGE_SIZE (4KB) for now, it would
>> be more correct to use Xen specific #define-s as XEN_PAGE_SIZE can
>> be changed at some point in the future.
>>
>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>> ---
>> Cc: Juergen Gross <jgross@xxxxxxxx>
>> Cc: Xenia Ragiadakou <burzalodowa@xxxxxxxxx>
>>
>> As it was proposed at:
>> https://urldefense.com/v3/__https://lore.kernel.org/xen-devel/20221005174823.1800761-1-olekstysh@xxxxxxxxx/__;!!GF_29dbcQIUBPA!zHt-xZ_7tZc_EM6zva21E_YgwIiEeimFWfsJIpPwAu-TBcnzQhXHqlKzmXmwIcI6uIx_arHNZiaZeHt_428_8p-DyMpd$
>>  
>> [lore[.]kernel[.]org]
>>
>> Should go in only after that series.
>> ---
>>   drivers/xen/grant-dma-ops.c | 20 ++++++++++----------
>>   1 file changed, 10 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
>> index c66f56d24013..5392fdc25dca 100644
>> --- a/drivers/xen/grant-dma-ops.c
>> +++ b/drivers/xen/grant-dma-ops.c
>> @@ -31,12 +31,12 @@ static DEFINE_XARRAY_FLAGS(xen_grant_dma_devices, 
>> XA_FLAGS_LOCK_IRQ);
>>     static inline dma_addr_t grant_to_dma(grant_ref_t grant)
>>   {
>> -    return XEN_GRANT_DMA_ADDR_OFF | ((dma_addr_t)grant << PAGE_SHIFT);
>> +    return XEN_GRANT_DMA_ADDR_OFF | ((dma_addr_t)grant << 
>> XEN_PAGE_SHIFT);
>>   }
>
> With this change, can the offset added to the dma handle, generated by 
> grant_to_dma(), be the offset in the page? Couldn't it corrupt the 
> grant ref?


Good point, indeed, I think it could corrupt if guest uses a different 
than Xen page granularity (i.e 64KB).


>
>>     static inline grant_ref_t dma_to_grant(dma_addr_t dma)
>>   {
>> -    return (grant_ref_t)((dma & ~XEN_GRANT_DMA_ADDR_OFF) >> 
>> PAGE_SHIFT);
>> +    return (grant_ref_t)((dma & ~XEN_GRANT_DMA_ADDR_OFF) >> 
>> XEN_PAGE_SHIFT);
>>   }
>>     static struct xen_grant_dma_data *find_xen_grant_dma_data(struct 
>> device *dev)
>> @@ -79,7 +79,7 @@ static void *xen_grant_dma_alloc(struct device 
>> *dev, size_t size,
>>                    unsigned long attrs)
>>   {
>>       struct xen_grant_dma_data *data;
>> -    unsigned int i, n_pages = PFN_UP(size);
>> +    unsigned int i, n_pages = XEN_PFN_UP(size);
>>       unsigned long pfn;
>>       grant_ref_t grant;
>>       void *ret;
>> @@ -91,14 +91,14 @@ static void *xen_grant_dma_alloc(struct device 
>> *dev, size_t size,
>>       if (unlikely(data->broken))
>>           return NULL;
>>   -    ret = alloc_pages_exact(n_pages * PAGE_SIZE, gfp);
>> +    ret = alloc_pages_exact(n_pages * XEN_PAGE_SIZE, gfp);
>>       if (!ret)
>>           return NULL;
>>         pfn = virt_to_pfn(ret);
>>         if (gnttab_alloc_grant_reference_seq(n_pages, &grant)) {
>> -        free_pages_exact(ret, n_pages * PAGE_SIZE);
>> +        free_pages_exact(ret, n_pages * XEN_PAGE_SIZE);
>>           return NULL;
>>       }
>>   @@ -116,7 +116,7 @@ static void xen_grant_dma_free(struct device 
>> *dev, size_t size, void *vaddr,
>>                      dma_addr_t dma_handle, unsigned long attrs)
>>   {
>>       struct xen_grant_dma_data *data;
>> -    unsigned int i, n_pages = PFN_UP(size);
>> +    unsigned int i, n_pages = XEN_PFN_UP(size);
>>       grant_ref_t grant;
>>         data = find_xen_grant_dma_data(dev);
>> @@ -138,7 +138,7 @@ static void xen_grant_dma_free(struct device 
>> *dev, size_t size, void *vaddr,
>>         gnttab_free_grant_reference_seq(grant, n_pages);
>>   -    free_pages_exact(vaddr, n_pages * PAGE_SIZE);
>> +    free_pages_exact(vaddr, n_pages * XEN_PAGE_SIZE);
>>   }
>>     static struct page *xen_grant_dma_alloc_pages(struct device *dev, 
>> size_t size,
>> @@ -168,7 +168,7 @@ 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 int i, n_pages = XEN_PFN_UP(offset + size);
>
> The offset, here, refers to the offset in the page ...
>
>>       grant_ref_t grant;
>>       dma_addr_t dma_handle;
>>   @@ -200,8 +200,8 @@ static void xen_grant_dma_unmap_page(struct 
>> device *dev, dma_addr_t dma_handle,
>>                        unsigned long attrs)
>>   {
>>       struct xen_grant_dma_data *data;
>> -    unsigned long offset = dma_handle & (PAGE_SIZE - 1);
>> -    unsigned int i, n_pages = PFN_UP(offset + size);
>> +    unsigned long offset = dma_handle & ~XEN_PAGE_MASK;
>
> ... while, here, it refers to the offset in the grant.
> So, the calculated number of grants may differ.

Good point, I think you are right, so we need to additionally use 
xen_offset_in_page() macro in xen_grant_dma_map_page(),

something like that to be squashed with current patch:


diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
index 9d5eca6d638a..bb984dc05deb 100644
--- a/drivers/xen/grant-dma-ops.c
+++ b/drivers/xen/grant-dma-ops.c
@@ -169,7 +169,7 @@ 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 = XEN_PFN_UP(offset + size);
+       unsigned int i, n_pages = XEN_PFN_UP(xen_offset_in_page(offset) 
+ size);
         grant_ref_t grant;
         dma_addr_t dma_handle;

@@ -191,7 +191,7 @@ static dma_addr_t xen_grant_dma_map_page(struct 
device *dev, struct page *page,
                                 xen_page_to_gfn(page) + i, dir == 
DMA_TO_DEVICE);
         }

-       dma_handle = grant_to_dma(grant) + offset;
+       dma_handle = grant_to_dma(grant) + xen_offset_in_page(offset);

         return dma_handle;
  }

Did I get your point right?


>
>
>> +    unsigned int i, n_pages = XEN_PFN_UP(offset + size);
>>       grant_ref_t grant;
>>         if (WARN_ON(dir == DMA_NONE))
>

Thank you.


-- 
Regards,

Oleksandr Tyshchenko