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

[Ashish Kalra <ashish.kalra@amd.com>]

Hello Konrad,

On Mon, Nov 23, 2020 at 12:56:32PM -0500, Konrad Rzeszutek Wilk wrote:
> On Mon, Nov 23, 2020 at 06:06:47PM +0100, Borislav Petkov wrote:
> > On Thu, Nov 19, 2020 at 09:42:05PM +0000, Ashish Kalra wrote:
> > > From: Ashish Kalra <ashish.kalra@amd.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@intel.com>
> > > 
> > > Signed-off-by: Ashish Kalra <ashish.kalra@amd.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);
> 
> 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. And then going forward we can allocate memory _after_ boot and then stich
> the late SWIOTLB pool and allocate on demand.
> 
> 
Ok. 

As i mentioned earlier, the patch was initially based on using a % of guest memory,
as below:

+#define SEV_ADJUST_SWIOTLB_SIZE_PERCENT        5
+#define SEV_ADJUST_SWIOTLB_SIZE_MAX    (1UL << 30)
...
...
+       if (sev_active() && !io_tlb_nslabs) {
+               unsigned long total_mem = get_num_physpages() <<
+ PAGE_SHIFT;
+
+               default_size = total_mem *
+                       SEV_ADJUST_SWIOTLB_SIZE_PERCENT / 100;
+
+               default_size = ALIGN(default_size, 1 << IO_TLB_SHIFT);
+
+               default_size = clamp_val(default_size, IO_TLB_DEFAULT_SIZE,
+                       SEV_ADJUST_SWIOTLB_SIZE_MAX);
+       }

So a similar logic can be applied here.

> 
> > > +		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.
> 
> This is not going to work for TDX. I think having a registration
> to SWIOTLB to have this function would be better going forward.
> 
> As in there will be a swiotlb_register_adjuster() which AMD SEV
> code can call at start, also TDX can do it (and other platforms).
> 

The question is how does mem_encrypt_init() work ?

That uses a similar logic as arch_swiotlb_adjust() as a "__weak"
function and i am sure it will also need to have added support for TDX,
can't both arch_swiotlb_adjust() and mem_encrypt_init() have specific
checks for active AMD/INTEL memory encryption technology and accordingly
perform actions, as mem_encrypt_init() currently checks for
sev_active().

init/main.c: 

void __init __weak mem_encrypt_init(void) { }

start_kernel()
{
   ..
   mem_encrypt_init();
   ..
}

arch/x86/mm/mem_encrypt.c: 
    
/* Architecture __weak replacement functions */

void __init mem_encrypt_init(void)
{
        if (!sme_me_mask)
                return;

        /* Call into SWIOTLB to update the SWIOTLB DMA buffers */
        swiotlb_update_mem_attributes();

        /*
         * With SEV, we need to unroll the rep string I/O instructions.
         */
        if (sev_active())
                static_branch_enable(&sev_enable_key);
...
...

Thanks,
Ashish

> > 
> > > +}
> > > +
> > > +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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fpeople.kernel.org%2Ftglx%2Fnotes-about-netiquette&amp;data=04%7C01%7CAshish.Kalra%40amd.com%7Cebd4a85f98f44bdfcb5408d88fd8dfac%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637417508926083910%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=Ub9PjAPzhDWr7K2iQggTAXwgg4VbORxP%2F%2Fcg6gQreCc%3D&amp;reserved=0
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