Messages in this thread Patch in this message | | | Date | Mon, 31 Jul 2017 18:15:30 +0800 | From | Baoquan He <> | Subject | Re: [PATCH v8 10/13] iommu/amd: Allocate memory below 4G for dev table if translation pre-enabled |
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Hi Joerg,
On 07/28/17 at 05:06pm, Baoquan He wrote: > Hi Joerg, > > On 07/27/17 at 05:55pm, Joerg Roedel wrote: > > On Fri, Jul 21, 2017 at 04:59:08PM +0800, Baoquan He wrote: > > > AMD pointed out it's unsafe to update the device-table while iommu > > > is enabled. It turns out that device-table pointer update is split > > > up into two 32bit writes in the IOMMU hardware. So updating it while > > > the IOMMU is enabled could have some nasty side effects. > > > > > > The only way to work around this is to allocate the device-table below > > > 4GB if translation is pre-enabled in kdump kernel. If allocation failed, > > > still use the old one. > > > > Not only for the kdump kernel. The old device table must also be below > > 4GB so that its pointer can be updated with a 32bit write. > > > > If the old table is above 4GB you still need the second write to zero > > the upper parts of the pointer in hardware. > > Do you mean the allocation of amd_iommu_dev_table in > early_amd_iommu_init() also need be addressed for 1st kernel? Seems we > don't make sure that for 1st kernel, like adding GFP_DMA32 flag when > allocate amd_iommu_dev_table in amd_iommu_dev_table > early_amd_iommu_init().
I plan to add GFP_DMA32 when allocate amd_iommu_dev_table in early_amd_iommu_init() as below. Then in kdump kernel we don't need to worry if the old amd_iommu_dev_table could be above 4G, right? And might not need to check if it's above 4G, right?
diff --git a/drivers/iommu/amd_iommu_init.c b/drivers/iommu/amd_iommu_init.c index 781a138..85d6445 100644 --- a/drivers/iommu/amd_iommu_init.c +++ b/drivers/iommu/amd_iommu_init.c @@ -2436,7 +2436,8 @@ static int __init early_amd_iommu_init(void) /* Device table - directly used by all IOMMUs */ ret = -ENOMEM; - amd_iommu_dev_table = (void *)__get_free_pages(GFP_KERNEL | __GFP_ZERO, + amd_iommu_dev_table = (void *)__get_free_pages( + GFP_KERNEL | __GFP_ZERO | GFP_DMA32, get_order(dev_table_size)); if (amd_iommu_dev_table == NULL) goto out; -- 2.9.4
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