linux-kernel - Re: [PATCH v6] swiotlb: Adjust SWIOTBL bounce buffer size for SEV guests.

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Date:   Mon, 23 Nov 2020 18:06:47 +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 v6] swiotlb: Adjust SWIOTBL bounce buffer size for SEV
 guests.

On Thu, Nov 19, 2020 at 09:42:05PM +0000, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@....com>
> 
> 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.
> 
> Increase the default size of SWIOTLB for SEV guests using a minimum
> value of 128MB and a maximum value of 512MB, determining on amount
> of provisioned guest memory.

That sentence needs massaging.

> 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 calling it explicitly from setup_arch().

"hence call it ... "

> 
> The SWIOTLB default size adjustment is added as an architecture specific

"... is added... " needs to be "Add ..."

> 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@...el.com>
> 
> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
> ---
>  arch/x86/kernel/setup.c   |  2 ++
>  arch/x86/mm/mem_encrypt.c | 32 ++++++++++++++++++++++++++++++++
>  include/linux/swiotlb.h   |  6 ++++++
>  kernel/dma/swiotlb.c      | 24 ++++++++++++++++++++++++
>  4 files changed, 64 insertions(+)
> 
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 3511736fbc74..b073d58dd4a3 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -1166,6 +1166,8 @@ 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 3f248f0d0e07..c79a0d761db5 100644
> --- a/arch/x86/mm/mem_encrypt.c
> +++ b/arch/x86/mm/mem_encrypt.c
> @@ -490,6 +490,38 @@ static void print_mem_encrypt_feature_info(void)
>  }
>  
>  /* Architecture __weak replacement functions */
> +unsigned long __init arch_swiotlb_adjust(unsigned long iotlb_default_size)
> +{
> +	unsigned long size = 0;

	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 & SWIOTLB may
						     ^

Use words pls, not "&".


> +	 * 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("SWIOTLB bounce buffer size adjusted to %luMB for SEV platform",

just "... for SEV" - no need for "platform".

...

> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index c19379fabd20..3be9a19ea0a5 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -163,6 +163,30 @@ 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 0;

That, of course, needs to return size, not 0.

> +}
> +
> +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.
> +	 */

And when you preset the function-local argument "size" with the size
coming in as the size argument of arch_swiotlb_adjust()...

> +	if (!io_tlb_nslabs) {
> +		size = arch_swiotlb_adjust(IO_TLB_DEFAULT_SIZE);
> +		if (size) {

... you don't have to do if (size) here either but simply use size to
compute io_tlb_nslabs, I'd say.

Thx.

-- 
Regards/Gruss,
    Boris.

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