linux-kernel - Re: [PATCH v1] xen/swiotlb: rework early repeat code

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Date:   Fri, 24 May 2019 13:58:50 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Sergey Dyasli <sergey.dyasli@...rix.com>
cc:     iommu@...ts.linux-foundation.org, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Paul Durrant <paul.durrant@...rix.com>
Subject: Re: [PATCH v1] xen/swiotlb: rework early repeat code

On Fri, 24 May 2019, Sergey Dyasli wrote:
> Current repeat code is plain broken for the early=true case. Xen exchanges
> all DMA (<4GB) pages that it can on the first xen_swiotlb_fixup() attempt.
> All further attempts with a halved region will fail immediately because
> all DMA pages already belong to Dom0.
> 
> Introduce contig_pages param for xen_swiotlb_fixup() to track the number
> of pages that were made contiguous in MFN space and use the same bootmem
> region while halving the memory requirements.
> 
> Signed-off-by: Sergey Dyasli <sergey.dyasli@...rix.com>

Just FYI I am touching the same code to fix another unrelated bug, see:

https://marc.info/?l=xen-devel&m=155856767022893


> ---
> CC: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> CC: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> CC: Juergen Gross <jgross@...e.com>
> CC: Stefano Stabellini <sstabellini@...nel.org>
> CC: Paul Durrant <paul.durrant@...rix.com>
> ---
>  drivers/xen/swiotlb-xen.c | 36 ++++++++++++++++++++++++++++++------
>  1 file changed, 30 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> index 5dcb06fe9667..d2aba804d06c 100644
> --- a/drivers/xen/swiotlb-xen.c
> +++ b/drivers/xen/swiotlb-xen.c
> @@ -142,7 +142,8 @@ static int is_xen_swiotlb_buffer(dma_addr_t dma_addr)
>  static int max_dma_bits = 32;
>  
>  static int
> -xen_swiotlb_fixup(void *buf, size_t size, unsigned long nslabs)
> +xen_swiotlb_fixup(void *buf, size_t size, unsigned long nslabs,
> +		  unsigned long *contig_pages)
>  {
>  	int i, rc;
>  	int dma_bits;
> @@ -156,10 +157,13 @@ xen_swiotlb_fixup(void *buf, size_t size, unsigned long nslabs)
>  		int slabs = min(nslabs - i, (unsigned long)IO_TLB_SEGSIZE);
>  
>  		do {
> +			unsigned int order = get_order(slabs << IO_TLB_SHIFT);
>  			rc = xen_create_contiguous_region(
>  				p + (i << IO_TLB_SHIFT),
> -				get_order(slabs << IO_TLB_SHIFT),
> +				order,
>  				dma_bits, &dma_handle);
> +			if (rc == 0)
> +				*contig_pages += 1 << order;
>  		} while (rc && dma_bits++ < max_dma_bits);
>  		if (rc)
>  			return rc;
> @@ -202,7 +206,7 @@ static const char *xen_swiotlb_error(enum xen_swiotlb_err err)
>  }
>  int __ref xen_swiotlb_init(int verbose, bool early)
>  {
> -	unsigned long bytes, order;
> +	unsigned long bytes, order, contig_pages;
>  	int rc = -ENOMEM;
>  	enum xen_swiotlb_err m_ret = XEN_SWIOTLB_UNKNOWN;
>  	unsigned int repeat = 3;
> @@ -244,13 +248,32 @@ int __ref xen_swiotlb_init(int verbose, bool early)
>  	/*
>  	 * And replace that memory with pages under 4GB.
>  	 */
> +	contig_pages = 0;
>  	rc = xen_swiotlb_fixup(xen_io_tlb_start,
>  			       bytes,
> -			       xen_io_tlb_nslabs);
> +			       xen_io_tlb_nslabs,
> +			       &contig_pages);
>  	if (rc) {
> -		if (early)
> +		if (early) {
> +			unsigned long orig_bytes = bytes;
> +			while (repeat-- > 0) {
> +				xen_io_tlb_nslabs = max(1024UL, /* Min is 2MB */
> +						      (xen_io_tlb_nslabs >> 1));
> +				pr_info("Lowering to %luMB\n",
> +				     (xen_io_tlb_nslabs << IO_TLB_SHIFT) >> 20);
> +				bytes = xen_set_nslabs(xen_io_tlb_nslabs);
> +				order = get_order(xen_io_tlb_nslabs << IO_TLB_SHIFT);
> +				xen_io_tlb_end = xen_io_tlb_start + bytes;
> +				if (contig_pages >= (1ul << order)) {
> +					/* Enough pages were made contiguous */
> +					memblock_free(__pa(xen_io_tlb_start + bytes),
> +						     PAGE_ALIGN(orig_bytes - bytes));
> +					goto fixup_done;
> +				}
> +			}
>  			memblock_free(__pa(xen_io_tlb_start),
>  				      PAGE_ALIGN(bytes));
> +		}
>  		else {
>  			free_pages((unsigned long)xen_io_tlb_start, order);
>  			xen_io_tlb_start = NULL;
> @@ -258,6 +281,7 @@ int __ref xen_swiotlb_init(int verbose, bool early)
>  		m_ret = XEN_SWIOTLB_EFIXUP;
>  		goto error;
>  	}
> +fixup_done:
>  	start_dma_addr = xen_virt_to_bus(xen_io_tlb_start);
>  	if (early) {
>  		if (swiotlb_init_with_tbl(xen_io_tlb_start, xen_io_tlb_nslabs,
> @@ -272,7 +296,7 @@ int __ref xen_swiotlb_init(int verbose, bool early)
>  
>  	return rc;
>  error:
> -	if (repeat--) {
> +	if (repeat-- > 0) {
>  		xen_io_tlb_nslabs = max(1024UL, /* Min is 2MB */
>  					(xen_io_tlb_nslabs >> 1));
>  		pr_info("Lowering to %luMB\n",
> -- 
> 2.17.1
>