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Date: Fri, 7 Oct 2022 18:50:33 +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: Convert PAGE_SIZE/PAGE_SHIFT/PFN_UP to Xen
counterparts
On 10/7/22 16:43, Oleksandr Tyshchenko wrote:
>
> On 07.10.22 10:15, Xenia Ragiadakou wrote:
>>
>> On 10/7/22 00:13, Oleksandr Tyshchenko wrote:
>>
>> Hi Oleksandr
>
>
> Hello Xenia
>
>
>>
>>>
>>> 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@...m.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@...m.com>
>>>>> ---
>>>>> Cc: Juergen Gross <jgross@...e.com>
>>>>> Cc: Xenia Ragiadakou <burzalodowa@...il.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);
>> }
>
>
> Thanks, valid point. Agree it's indeed more complicated. I will comment
> on that later. I have just pushed another fix, it is not related to
> XEN_PAGE_SIZE directly, but also about page offset > PAGE_SIZE
>
I got a little bit confused with the order that the patches will be
applied :)
IIUC the above scenario cannot happen, i.e the offset to be > PAGE_SIZE,
because this callback is used to map for transfer a portion of a single
page.
> so touches the same code and should be prereq:
>
> https://lore.kernel.org/all/20221007132736.2275574-1-olekstysh@gmail.com/
>
>
>>
>>>>
>>>>
>>>>> + unsigned int i, n_pages = XEN_PFN_UP(offset + size);
>>>>> grant_ref_t grant;
>>>>> if (WARN_ON(dir == DMA_NONE))
>>>>
>>>
>>> Thank you.
>>>
>>>
>>
--
Xenia
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