Messages in this thread | | | Date | Wed, 15 Feb 2023 11:09:22 +0000 | Subject | Re: [PATCH 2/4] iommu: Use group ownership to avoid driver attachment | From | Robin Murphy <> |
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On 2023-02-15 07:28, Baolu Lu wrote: > On 2023/2/15 14:56, Tian, Kevin wrote: >>> From: Baolu Lu<baolu.lu@linux.intel.com> >>> Sent: Wednesday, February 15, 2023 1:51 PM >>> >>> On 2/13/23 10:19 PM, Jason Gunthorpe wrote: >>>> On Mon, Feb 13, 2023 at 03:49:39PM +0800, Lu Baolu wrote: >>>>> @@ -2992,6 +2987,14 @@ static ssize_t iommu_group_store_type(struct >>> iommu_group *group, >>>>> else >>>>> return -EINVAL; >>>>> >>>>> + if (req_type != IOMMU_DOMAIN_DMA_FQ || >>>>> + group->default_domain->type != IOMMU_DOMAIN_DMA) { >>>>> + ret = iommu_group_claim_dma_owner(group, (void *)buf); >>>>> + if (ret) >>>>> + return ret; >>>>> + group_owner_claimed = true; >>>>> + } >>>> I don't get it, this should be done unconditionally. If we couldn't >>>> take ownership then we simply can't progress. >>> The existing code allows the user to switch the default domain from >>> strict to lazy invalidation mode. The default domain is not changed, >>> hence it should be seamless and transparent to the device driver. >> Is there real usage relying on this transition for a bound device? >> >> In concept strict->lazy transition implies relaxed DMA security. It's >> hard >> to think of a motivation of doing so while the device might be doing >> in-fly DMAs. >> >> Presumably such perf/security tradeoff should be planned way before >> binding device/driver together. >> >> btw if strict->lazy is allowed why lazy->strict is prohibited? >> > > We all know, strict vs. lazy is a tradeoff between performance and > security. > > strict -> lazy: driver works in secure mode. This transition trades off > security for better performance. > > lazy->strict: The driver is already working in non-safety mode. This > transition only results in worse performance. It makes no sense. If user > want to put the driver in a secure mode, they need to unbind the driver, > reset the device and do the lazy->strict transition. > > Robin might have better insights.
Yes, this was added for a definite use-case in ChromeOS, where strict->lazy needs to support being done "live" since the device in question is the storage controller for the already-mounted root filesystem. Your reasoning seems to match what I summarised in the original commit message :)
Thanks, Robin.
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