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Message-ID: <20210122132657.GF24102@willie-the-truck>
Date: Fri, 22 Jan 2021 13:26:57 +0000
From: Will Deacon <will@...nel.org>
To: "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
Cc: Robin Murphy <robin.murphy@....com>,
Joerg Roedel <joro@...tes.org>,
Jean-Philippe Brucker <jean-philippe@...aro.org>,
Jonathan Cameron <Jonathan.Cameron@...wei.com>,
linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
iommu <iommu@...ts.linux-foundation.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Yang Yingliang <yangyingliang@...wei.com>
Subject: Re: [PATCH 1/1] iommu/arm-smmu-v3: add support for BBML
On Fri, Jan 22, 2021 at 09:21:20PM +0800, Leizhen (ThunderTown) wrote:
> On 2021/1/22 21:00, Robin Murphy wrote:
> > On 2021-01-22 12:51, Will Deacon wrote:
> >> On Thu, Nov 26, 2020 at 11:42:30AM +0800, Zhen Lei wrote:
> >>> - tblp = ptep - ARM_LPAE_LVL_IDX(iova, lvl, data);
> >>> - if (__arm_lpae_unmap(data, NULL, iova, sz, lvl, tblp) != sz) {
> >>> - WARN_ON(1);
> >>> - return -EINVAL;
> >>> + switch (cfg->bbml) {
> >>> + case 0:
> >>> + /*
> >>> + * We need to unmap and free the old table before
> >>> + * overwriting it with a block entry.
> >>> + */
> >>> + tblp = ptep - ARM_LPAE_LVL_IDX(iova, lvl, data);
> >>> + if (__arm_lpae_unmap(data, NULL, iova, sz, lvl, tblp) != sz) {
> >>> + WARN_ON(1);
> >>> + return -EINVAL;
> >>> + }
> >>> + break;
> >>> + case 1:
> >>> + __arm_lpae_init_pte(data, paddr, prot, lvl, ptep, ARM_LPAE_PTE_nT);
> >>> +
> >>> + io_pgtable_tlb_flush_walk(iop, iova, sz, ARM_LPAE_GRANULE(data));
> >>> + tblp = iopte_deref(pte, data);
> >>> + __arm_lpae_free_pgtable(data, lvl + 1, tblp);
> >>> + break;
> >>> + case 2:
> >>> + __arm_lpae_init_pte(data, paddr, prot, lvl, ptep, 0);
> >>> +
> >>> + io_pgtable_tlb_flush_walk(iop, iova, sz, ARM_LPAE_GRANULE(data));
> >>> + tblp = iopte_deref(pte, data);
> >>> + __arm_lpae_free_pgtable(data, lvl + 1, tblp);
> >>> + return 0;
> >>
> >> Sorry, but I really don't understand what you're trying to do here. The old
> >> code uses BBM for the table -> block path so we don't need anything extra
> >> here. The dodgy case is when we unmap part of a block, and end up installing
> >> a table via arm_lpae_split_blk_unmap(). We can't use BBM there because there
> >> could be ongoing DMA to parts of the block mapping that we want to remain in
> >> place.
> >>
> >> Are you seeing a problem in practice?
> >
> > Right, I was under the assumption that we could ignore BBML because we
> > should never have a legitimate reason to split blocks. I'm certainly not
> > keen on piling any more complexity into split_blk_unmap, because the
> > IOMMU API clearly doesn't have a well-defined behaviour for that case
> > anyway - some other drivers will just unmap the entire block, and IIRC
> > there was a hint somewhere in VFIO that it might actually expect that
> > behaviour.
>
> I'm going home. I'll answer you two tomorrow.
It can wait until Monday! Have a good weekend :)
Will
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