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

[Ashish Kalra]

Hello Boris, 

On Mon, Dec 07, 2020 at 01:10:07PM +0100, Borislav Petkov wrote:
> 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?
>

This is related to the earlier static adjustment of the SWIOTLB buffers
as per guest memory size and Konrad's feedback on the same, as copied
below : 

>>That is eating 128MB for 1GB, aka 12% of the guest memory allocated statically for this.
>> 
>> And for guests that are 2GB, that is 12% until it gets to 3GB when 
>> it is 8% and then 6% at 4GB.
>> 
>> I would prefer this to be based on your memory count, that is 6% of 
>> total memory. 

Thanks,
Ashish

> > +#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.
> 
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