linux-kernel - Re: [PATCH v5] swiotlb: Adjust SWIOTBL bounce buffer size for SEV guests.

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Message-ID: <20201118223916.GP7472@zn.tnic>
Date:   Wed, 18 Nov 2020 23:39:16 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Ashish Kalra <Ashish.Kalra@....com>
Cc:     konrad.wilk@...cle.com, hch@....de, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com, x86@...nel.org, luto@...nel.org,
        peterz@...radead.org, dave.hansen@...ux-intel.com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        brijesh.singh@....com, Thomas.Lendacky@....com, jon.grimm@....com,
        rientjes@...gle.com
Subject: Re: [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