Re: [PATCH v2 5/8] iommu/io-pgtable-arm: Support lockless operation

Linu Cherian
2017-06-23T11:34:05Z

On Fri Jun 23, 2017 at 11:35:25AM +0100, Robin Murphy wrote:
> On 23/06/17 09:56, Linu Cherian wrote:
> > On Fri Jun 23, 2017 at 11:23:26AM +0530, Linu Cherian wrote:
> >>
> >> Robin,
> >> Was trying to understand the new changes. Had few questions on
> >> arm_lpae_install_table.
> >>
> >> On Thu Jun 22, 2017 at 04:53:54PM +0100, Robin Murphy wrote:
> >>> For parallel I/O with multiple concurrent threads servicing the same
> >>> device (or devices, if several share a domain), serialising page table
> >>> updates becomes a massive bottleneck. On reflection, though, we don't
> >>> strictly need to do that - for valid IOMMU API usage, there are in fact
> >>> only two races that we need to guard against: multiple map requests for
> >>> different blocks within the same region, when the intermediate-level
> >>> table for that region does not yet exist; and multiple unmaps of
> >>> different parts of the same block entry. Both of those are fairly easily
> >>> solved by using a cmpxchg to install the new table, such that if we then
> >>> find that someone else's table got there first, we can simply free ours
> >>> and continue.
> >>>
> >>> Make the requisite changes such that we can withstand being called
> >>> without the caller maintaining a lock. In theory, this opens up a few
> >>> corners in which wildly misbehaving callers making nonsensical
> >>> overlapping requests might lead to crashes instead of just unpredictable
> >>> results, but correct code really does not deserve to pay a significant
> >>> performance cost for the sake of masking bugs in theoretical broken code.
> >>>
> >>> Signed-off-by: Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>
> >>> ---
> >>>
> >>> v2:
> >>> - Fix barriers in install_table
> >>> - Make a few more PTE accesses with {READ,WRITE}_ONCE() just in case
> >>> - Minor cosmetics
> >>>
> >>> drivers/iommu/io-pgtable-arm.c | 72 +++++++++++++++++++++++++++++++++---------
> >>> 1 file changed, 57 insertions(+), 15 deletions(-)
> >>>
> >>> diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
> >>> index 6334f51912ea..52700fa958c2 100644
> >>> --- a/drivers/iommu/io-pgtable-arm.c
> >>> +++ b/drivers/iommu/io-pgtable-arm.c
> >>> @@ -20,6 +20,7 @@
> >>>
> >>> #define pr_fmt(fmt) "arm-lpae io-pgtable: " fmt
> >>>
> >>> +#include <linux/atomic.h>
> >>> #include <linux/iommu.h>
> >>> #include <linux/kernel.h>
> >>> #include <linux/sizes.h>
> >>> @@ -99,6 +100,8 @@
> >>> #define ARM_LPAE_PTE_ATTR_HI_MASK (((arm_lpae_iopte)6) << 52)
> >>> #define ARM_LPAE_PTE_ATTR_MASK (ARM_LPAE_PTE_ATTR_LO_MASK | \
> >>> ARM_LPAE_PTE_ATTR_HI_MASK)
> >>> +/* Software bit for solving coherency races */
> >>> +#define ARM_LPAE_PTE_SW_SYNC (((arm_lpae_iopte)1) << 55)
> >>>
> >>> /* Stage-1 PTE */
> >>> #define ARM_LPAE_PTE_AP_UNPRIV (((arm_lpae_iopte)1) << 6)
> >>> @@ -249,15 +252,20 @@ static void __arm_lpae_free_pages(void *pages, size_t size,
> >>> free_pages_exact(pages, size);
> >>> }
> >>>
> >>> +static void __arm_lpae_sync_pte(arm_lpae_iopte *ptep,
> >>> + struct io_pgtable_cfg *cfg)
> >>> +{
> >>> + dma_sync_single_for_device(cfg->iommu_dev, __arm_lpae_dma_addr(ptep),
> >>> + sizeof(*ptep), DMA_TO_DEVICE);
> >>> +}
> >>> +
> >>> static void __arm_lpae_set_pte(arm_lpae_iopte *ptep, arm_lpae_iopte pte,
> >>> struct io_pgtable_cfg *cfg)
> >>> {
> >>> *ptep = pte;
> >>>
> >>> if (!(cfg->quirks & IO_PGTABLE_QUIRK_NO_DMA))
> >>> - dma_sync_single_for_device(cfg->iommu_dev,
> >>> - __arm_lpae_dma_addr(ptep),
> >>> - sizeof(pte), DMA_TO_DEVICE);
> >>> + __arm_lpae_sync_pte(ptep, cfg);
> >>> }
> >>>
> >>> static int __arm_lpae_unmap(struct arm_lpae_io_pgtable *data,
> >>> @@ -314,16 +322,30 @@ static int arm_lpae_init_pte(struct arm_lpae_io_pgtable *data,
> >>>
> >>> static arm_lpae_iopte arm_lpae_install_table(arm_lpae_iopte *table,
> >>> arm_lpae_iopte *ptep,
> >>> + arm_lpae_iopte curr,
> >>> struct io_pgtable_cfg *cfg)
> >>> {
> >>> - arm_lpae_iopte new;
> >>> + arm_lpae_iopte old, new;
> >>>
> >>> new = __pa(table) | ARM_LPAE_PTE_TYPE_TABLE;
> >>> if (cfg->quirks & IO_PGTABLE_QUIRK_ARM_NS)
> >>> new |= ARM_LPAE_PTE_NSTABLE;
> >>>
> >>> - __arm_lpae_set_pte(ptep, new, cfg);
> >>> - return new;
> >>> + /* Ensure the table itself is visible before its PTE can be */
> >>> + wmb();
> >>
> >> Could you please give more hints on why this is required.
>
> In theory it's possible for the write to ptep to become visible before
> the previous writes filling out the PTEs in table - if the IOMMU
> happened to speculatively walk ptep while parts of table were still sat
> in a write buffer somewhere, it could see the old contents of that page
> and potentially allocate bogus TLB entries if the stale data happened to
> look like valid PTEs. In the non-coherent case the DMA cache maintenance
> is sufficient, but otherwise we need a barrier to order writing the
> next-level PTEs strictly before writing the table PTE pointing to them,
> such that the IOMMU cannot at any point see a partially-initialised table.

Got it. Thanks a lot for explainig that.

>
> >>> +
> >>> + old = cmpxchg64_relaxed(ptep, curr, new);
> >>> +
> >>> + if ((cfg->quirks & IO_PGTABLE_QUIRK_NO_DMA) ||
> >>> + (old & ARM_LPAE_PTE_SW_SYNC))
> >>> + return old;
> >>> +
> >>> + /* Even if it's not ours, there's no point waiting; just kick it */
> >>> + __arm_lpae_sync_pte(ptep, cfg);
> >>> + if (old == curr)
> >>> + WRITE_ONCE(*ptep, new | ARM_LPAE_PTE_SW_SYNC);
> >>
> >> How is it ensured that the cache operations are completed before we flag them with
> >> ARM_LPAE_PTE_SW_SYNC. The previous version had a wmb() after the sync operation.
>
> The wmb() here was a silly oversight on my part - as Will reminded me,
> dma_sync_*() already has its own barrier to ensure completion, which is
> pretty obvious in retrospect because the entire streaming DMA would be
> totally broken otherwise.
>
> >>> +
> >>> + return old;
> >>> }
> >>>
> >>
> >
> >
> > Observed a performance drop of close to 1G,
> > while testing on the 10G network interface with this patch series compared to v1.
>
> Is that consistent over multiple runs? I wouldn't expect many workloads
> to be thrashing table and hugepage mappings in the same IOVA region, so
> after a point we should tend to reach a fairly steady state where we're
> only changing leaf PTEs.
>
> > Moving the wmb() as in v1 restores it back.
>
> Note that on a coherent platform like ThunderX that's as good as just
> deleting it, because you'll never execute the case below. However, on
> reflection I think it can at least safely be downgraded to dma_wmb()
> (i.e. DMB) rather than a full DSB - would you be able to test what
> difference that makes?

The testing was done on Thunderx 1, which has a non coherent page table walk.
Yes, downgrading to dma_wmb() helps. With this change, performance is back to v1.

Thanks.

--
Linu cherian