linux-kernel - Re: [PATCH] xen/virtio: Convert PAGE_SIZE/PAGE_SHIFT/PFN

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Message-ID: <96a16b32-0950-b538-65e5-9955ed8cc529@gmail.com>
Date:   Thu, 6 Oct 2022 20:59:33 +0300
From:   Xenia Ragiadakou <burzalodowa@...il.com>
To:     Oleksandr Tyshchenko <olekstysh@...il.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc:     Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH] xen/virtio: Convert PAGE_SIZE/PAGE_SHIFT/PFN_UP to Xen
 counterparts


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://lore.kernel.org/xen-devel/20221005174823.1800761-1-olekstysh@gmail.com/
> 
> 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?

>   
>   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.

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

-- 
Xenia