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

From: Ashish Kalra
Date: Tue Nov 24 2020 - 18:48:52 EST

Next message: Miguel Ojeda: "Re: [PATCH 000/141] Fix fall-through warnings for Clang"
Previous message: Geoff Levand: "Re: [PATCH v2 02/24] exec: Simplify unshare_files"
In reply to: Kalra, Ashish: "Re: [PATCH v6] swiotlb: Adjust SWIOTBL bounce buffer size for SEV guests."
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]

Hello Konrad,

On Mon, Nov 23, 2020 at 10:56:31PM +0000, Ashish Kalra wrote:
> 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@xxxxxxx>
> > > >
> > > > 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@xxxxxxxxx>
> > > >
> > > > Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> > > > ---
> > > > 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);
> ...
> ...
>

Your thoughts on this ?

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&data=04%7C01%7CAshish.Kalra%40amd.com%7Cebd4a85f98f44bdfcb5408d88fd8dfac%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637417508926083910%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=Ub9PjAPzhDWr7K2iQggTAXwgg4VbORxP%2F%2Fcg6gQreCc%3D&reserved=0

Next message: Miguel Ojeda: "Re: [PATCH 000/141] Fix fall-through warnings for Clang"
Previous message: Geoff Levand: "Re: [PATCH v2 02/24] exec: Simplify unshare_files"
In reply to: Kalra, Ashish: "Re: [PATCH v6] swiotlb: Adjust SWIOTBL bounce buffer size for SEV guests."
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]