Re: [PATCH v8] swiotlb: Adjust SWIOTBL bounce buffer size for SEV guests.

[Konrad Rzeszutek Wilk]

On Mon, Dec 07, 2020 at 11:10:57PM +0000, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@xxxxxxx>
> 
> For SEV, all DMA to and from guest has to use shared (un-encrypted) pages.
> SEV uses SWIOTLB to make this happen without requiring changes to device
> drivers.  However, depending on workload being run, the default 64MB of
> SWIOTLB might not be enough and SWIOTLB may run out of buffers to use
> for DMA, resulting in I/O errors and/or performance degradation for
> high I/O workloads.
> 
> Adjust the default size of SWIOTLB for SEV guests using a
> percentage of the total memory available to guest for SWIOTLB buffers.
> 
> Using late_initcall() interface to invoke swiotlb_adjust() does not
> work as the size adjustment needs to be done before mem_encrypt_init()
> and reserve_crashkernel() which use the allocated SWIOTLB buffer size,
> hence call it explicitly from setup_arch().
> 
> The SWIOTLB default size adjustment needs to be added as an architecture
> specific interface/callback to allow architectures such as those supporting
> memory encryption to adjust/expand SWIOTLB size for their use.
> 
> v5 fixed build errors and warnings as
> Reported-by: kbuild test robot <lkp@xxxxxxxxx>
> 
> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> ---
>  arch/x86/kernel/setup.c   |  2 ++
>  arch/x86/mm/mem_encrypt.c | 37 +++++++++++++++++++++++++++++++++++++
>  include/linux/swiotlb.h   |  6 ++++++
>  kernel/dma/swiotlb.c      | 22 ++++++++++++++++++++++
>  4 files changed, 67 insertions(+)
> 
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 84f581c91db4..31e24e198061 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -1149,6 +1149,8 @@ void __init setup_arch(char **cmdline_p)
>  	if (boot_cpu_has(X86_FEATURE_GBPAGES))
>  		hugetlb_cma_reserve(PUD_SHIFT - PAGE_SHIFT);
>  
> +	swiotlb_adjust();
> +
>  	/*
>  	 * Reserve memory for crash kernel after SRAT is parsed so that it
>  	 * won't consume hotpluggable memory.
> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
> index 1bcfbcd2bfd7..d1b8d60040cf 100644
> --- a/arch/x86/mm/mem_encrypt.c
> +++ b/arch/x86/mm/mem_encrypt.c
> @@ -485,7 +485,44 @@ static void print_mem_encrypt_feature_info(void)
>  	pr_cont("\n");
>  }
>  
> +/*
> + * The percentage of guest memory used here for SWIOTLB buffers
> + * is more of an approximation of the static adjustment which
> + * is 128M for <1G guests, 256M for 1G-4G guests and 512M for >4G guests.

No?

it is 64MB for <1G, and ~128M to 256M for 1G-to-4G

I will fix it up.
> + */
> +#define SEV_ADJUST_SWIOTLB_SIZE_PERCENT	6
> +
>  /* Architecture __weak replacement functions */
> +unsigned long __init arch_swiotlb_adjust(unsigned long iotlb_default_size)
> +{
> +	unsigned long size = iotlb_default_size;
> +
> +	/*
> +	 * For SEV, all DMA has to occur via shared/unencrypted pages.
> +	 * SEV uses SWOTLB to make this happen without changing device
> +	 * drivers. However, depending on the workload being run, the
> +	 * default 64MB of SWIOTLB may not be enough and`SWIOTLB may
> +	 * run out of buffers for DMA, resulting in I/O errors and/or
> +	 * performance degradation especially with high I/O workloads.
> +	 * Adjust the default size of SWIOTLB for SEV guests using
> +	 * a percentage of guest memory for SWIOTLB buffers.
> +	 * Also as the SWIOTLB bounce buffer memory is allocated
> +	 * from low memory, ensure that the adjusted size is within
> +	 * the limits of low available memory.
> +	 *
> +	 */
> +	if (sev_active()) {
> +		phys_addr_t total_mem = memblock_phys_mem_size();
> +
> +		size = total_mem * SEV_ADJUST_SWIOTLB_SIZE_PERCENT / 100;
> +		size = clamp_val(size, iotlb_default_size, SZ_1G);
> +		pr_info("SWIOTLB bounce buffer size adjusted to %luMB for SEV",
> +			size >> 20);
> +	}
> +
> +	return size;
> +}
> +
>  void __init mem_encrypt_init(void)
>  {
>  	if (!sme_me_mask)
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index 3bb72266a75a..b5904fa4b67c 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -33,6 +33,7 @@ extern void swiotlb_init(int verbose);
>  int swiotlb_init_with_tbl(char *tlb, unsigned long nslabs, int verbose);
>  extern unsigned long swiotlb_nr_tbl(void);
>  unsigned long swiotlb_size_or_default(void);
> +unsigned long __init arch_swiotlb_adjust(unsigned long size);
>  extern int swiotlb_late_init_with_tbl(char *tlb, unsigned long nslabs);
>  extern int swiotlb_late_init_with_default_size(size_t default_size);
>  extern void __init swiotlb_update_mem_attributes(void);
> @@ -77,6 +78,7 @@ void __init swiotlb_exit(void);
>  unsigned int swiotlb_max_segment(void);
>  size_t swiotlb_max_mapping_size(struct device *dev);
>  bool is_swiotlb_active(void);
> +void __init swiotlb_adjust(void);
>  #else
>  #define swiotlb_force SWIOTLB_NO_FORCE
>  static inline bool is_swiotlb_buffer(phys_addr_t paddr)
> @@ -99,6 +101,10 @@ static inline bool is_swiotlb_active(void)
>  {
>  	return false;
>  }
> +
> +static inline void swiotlb_adjust(void)
> +{
> +}
>  #endif /* CONFIG_SWIOTLB */
>  
>  extern void swiotlb_print_info(void);
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 781b9dca197c..0150ca2336bc 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -163,6 +163,28 @@ unsigned long swiotlb_size_or_default(void)
>  	return size ? size : (IO_TLB_DEFAULT_SIZE);
>  }
>  
> +unsigned long __init __weak arch_swiotlb_adjust(unsigned long size)
> +{
> +	return size;
> +}
> +
> +void __init swiotlb_adjust(void)
> +{
> +	unsigned long size;
> +
> +	/*
> +	 * If swiotlb parameter has not been specified, give a chance to
> +	 * architectures such as those supporting memory encryption to
> +	 * adjust/expand SWIOTLB size for their use.
> +	 */
> +	if (!io_tlb_nslabs) {
> +		size = arch_swiotlb_adjust(IO_TLB_DEFAULT_SIZE);
> +		size = ALIGN(size, 1 << IO_TLB_SHIFT);
> +		io_tlb_nslabs = size >> IO_TLB_SHIFT;
> +		io_tlb_nslabs = ALIGN(io_tlb_nslabs, IO_TLB_SEGSIZE);
> +	}
> +}
> +
>  void swiotlb_print_info(void)
>  {
>  	unsigned long bytes = io_tlb_nslabs << IO_TLB_SHIFT;
> -- 
> 2.17.1
>