linux-kernel - Re: [PATCH v7] swiotlb: Adjust SWIOTBL bounce buffer size for SEV guests.

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Date:   Mon, 7 Dec 2020 13:10:07 +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 v7] swiotlb: Adjust SWIOTBL bounce buffer size for SEV
 guests.

On Thu, Dec 03, 2020 at 03:25:59AM +0000, Ashish Kalra wrote:
> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
> index 1bcfbcd2bfd7..46549bd3d840 100644
> --- a/arch/x86/mm/mem_encrypt.c
> +++ b/arch/x86/mm/mem_encrypt.c
> @@ -485,7 +485,38 @@ static void print_mem_encrypt_feature_info(void)
>  	pr_cont("\n");
>  }

Any text about why 6% was chosen? A rule of thumb or so? Measurements?

> +#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();

Please integrate scripts/checkpatch.pl into your patch creation
workflow. Some of the warnings/errors *actually* make sense:

WARNING: Missing a blank line after declarations
#95: FILE: arch/x86/mm/mem_encrypt.c:511:
+               phys_addr_t total_mem = memblock_phys_mem_size();
+               size = total_mem * SEV_ADJUST_SWIOTLB_SIZE_PERCENT / 100;

But no need to resend now - just a hint for the future.

Konrad, ack?

On a 2G guest here, it says:

[    0.018373] SWIOTLB bounce buffer size adjusted to 122MB for SEV

so it makes sense to me.

Thx.

-- 
Regards/Gruss,
    Boris.

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