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Message-ID: <7f54bdab-c68f-0d38-93f4-007408151f01@gmail.com>
Date:   Fri, 7 Oct 2022 10:15:07 +0300
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Subject: Re: [PATCH] xen/virtio: Convert PAGE_SIZE/PAGE_SHIFT/PFN_UP to Xen
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To:     Oleksandr Tyshchenko <Oleksandr_Tyshchenko@epam.com>,
        "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Cc:     Stefano Stabellini <sstabellini@kernel.org>,
        Juergen Gross <jgross@suse.com>,
        Oleksandr Tyshchenko <olekstysh@gmail.com>
References: <20221006120912.1948459-1-olekstysh@gmail.com>
 <96a16b32-0950-b538-65e5-9955ed8cc529@gmail.com>
 <b3b8047e-b4a5-1e75-2a55-a7beecf8ca7d@epam.com>
From:   Xenia Ragiadakou <burzalodowa@gmail.com>
In-Reply-To: <b3b8047e-b4a5-1e75-2a55-a7beecf8ca7d@epam.com>
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On 10/7/22 00:13, Oleksandr Tyshchenko wrote:

Hi Oleksandr

> 
> 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@epam.com>
>>>
>>> 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@epam.com>
>>> ---
>>> Cc: Juergen Gross <jgross@suse.com>
>>> Cc: Xenia Ragiadakou <burzalodowa@gmail.com>
>>>
>>> As it was proposed at:
>>> https://urldefense.com/v3/__https://lore.kernel.org/xen-devel/20221005174823.1800761-1-olekstysh@gmail.com/__;!!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?
> 

I think it 's more complicated than that.
Let's say that the offset in page is > XEN_PAGE_SIZE, then the 
calculation of the number of grants won't take into account the part of 
the offset that is multiple of the XEN_PAGE_SIZE i.e it will calculate 
only the strictly necessary number of grants.
But xen_grant_dma_map_page() grants access to the whole page because, as 
it can be observed in the code snippet below, it does not take into 
account the page offset.

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);
}

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

-- 
Xenia