On Tue, Nov 03, 2020 at 11:00:27AM +0800, Kunkun Jiang wrote:
> Recently, I have read and learned the code related to io-pgtable-arm.c.
> There
> are two question on arm_lpae_install_table.
>
> 1、the first
>
> > 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 old, new;
> >
> > new = __pa(table) | ARM_LPAE_PTE_TYPE_TABLE;
> > if (cfg->quirks & IO_PGTABLE_QUIRK_ARM_NS)
> > new |= ARM_LPAE_PTE_NSTABLE;
> >
> > /*
> > * Ensure the table itself is visible before its PTE can be.
> > * Whilst we could get away with cmpxchg64_release below, this
> > * doesn't have any ordering semantics when !CONFIG_SMP.
> > */
> > dma_wmb();
> >
> > old = cmpxchg64_relaxed(ptep, curr, new);
> >
> > if (cfg->coherent_walk || (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);
> >
> > return old;
> > }
>
> If another thread changes the ptep between cmpxchg64_relaxed and
> WRITE_ONCE(*ptep, new | ARM_LPAE_PTE_SW_SYNC), the operation
> WRITE_ONCE will overwrite the change.
Can you please provide an example of a code path where this happens? The
idea is that CPUs can race on the cmpxchg(), but there will only be one
winner.
> 2、the second
>
> > for (i = 0; i < tablesz / sizeof(pte); i++, blk_paddr += split_sz) {
> > /* Unmap! */
> > if (i == unmap_idx)
> > continue;
> >
> > __arm_lpae_init_pte(data, blk_paddr, pte, lvl,
> > &tablep[i]);
> > }
> >
> > pte = arm_lpae_install_table(tablep, ptep, blk_pte, cfg);
>
> When altering a translation table descriptor include split a block into
> constituent granules, the Armv8-A and SMMUv3 architectures require
> a break-before-make procedure. But in the function arm_lpae_split_blk_unmap,
> it changes a block descriptor to an equivalent span of page translations
> directly. Is it appropriate to do so?
Break-before-make doesn't really work for the SMMU because faults are
generally fatal.
Are you seeing problems in practice with this code?
Will
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