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SubjectRe: [PATCH v5] swiotlb: Adjust SWIOTBL bounce buffer size for SEV guests.
On Wed, Nov 18, 2020 at 08:12:43PM +0000, Ashish Kalra wrote:
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 3511736fbc74..0f42911cea57 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -1166,6 +1166,10 @@ void __init setup_arch(char **cmdline_p)
> if (boot_cpu_has(X86_FEATURE_GBPAGES))
> hugetlb_cma_reserve(PUD_SHIFT - PAGE_SHIFT);
>
> +#ifdef CONFIG_X86_64
> + swiotlb_adjust();
> +#endif

Add an empty stub in include/linux/swiotlb.h for the !CONFIG_SWIOTLB
case and get rid of the ifdeffery please.

> +unsigned long __init arch_swiotlb_adjust(unsigned long iotlb_default_size)
> +{
> + unsigned long size = 0;
> +
> + /*
> + * 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 & SWIOTLB may
> + * run out of buffers for DMA, resulting in I/O errors and/or
> + * performance degradation especially with high I/O workloads.
> + * Increase the default size of SWIOTLB for SEV guests using
> + * a minimum value of 128MB and a maximum value of 512MB,
> + * depending on amount of provisioned guest memory.
> + */
> + if (sev_active()) {
> + phys_addr_t total_mem = memblock_phys_mem_size();
> +
> + if (total_mem <= SZ_1G)
> + size = max(iotlb_default_size, (unsigned long) SZ_128M);
> + else if (total_mem <= SZ_4G)
> + size = max(iotlb_default_size, (unsigned long) SZ_256M);
> + else
> + size = max(iotlb_default_size, (unsigned long) SZ_512M);
> +
> + pr_info("SEV adjusted max SWIOTLB size = %luMB",

Please make that message more user-friendly.

...

> +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);
> + if (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);
> +
> + pr_info("architecture adjusted SWIOTLB slabs = %lu\n",

That one too: what does "architecture adjusted SWIOTLB slabs" even
mean?!
Put yourself in your code user's shoes and see if that message makes
sense to her/him.

Thx.

--
Regards/Gruss,
Boris.

https://people.kernel.org/tglx/notes-about-netiquette

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