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Date:   Tue, 24 Nov 2020 23:46:22 +0000
From:   Ashish Kalra <ashish.kalra@amd.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc:     Borislav Petkov <bp@alien8.de>, hch@lst.de, tglx@linutronix.de,
        mingo@redhat.com, hpa@zytor.com, x86@kernel.org, luto@kernel.org,
        peterz@infradead.org, dave.hansen@linux-intel.com,
        iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        brijesh.singh@amd.com, Thomas.Lendacky@amd.com, jon.grimm@amd.com,
        rientjes@google.com
Subject: Re: [PATCH v6] swiotlb: Adjust SWIOTBL bounce buffer size for SEV
 guests.
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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@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);
> ...
> ...
> 

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&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