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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