2.12.2.2 SEV-SNP COMPLETION_WAIT Store Restrictions On systems that are SNP-enabled, the store address associated with any host
COMPLETION_WAIT command (s=1) is restricted. The Store Address must fall within the address range specified by the Completion Store
Base and Completion Store Limit registers. When the system is SNP-enabled, the memory within this range will be marked in the RMP
using a special immutable state by the PSP. This memory region will be readable by the CPU but not writable.
2.12.2.3 SEV-SNP Exclusion Range Restrictions The exclusion range feature is not supported on systems that are SNP-enabled.
Additionally, the Exclusion Base and Exclusion Range Limit registers are re-purposed to act as the Completion Store Base and Limit
registers.
Therefore, we need to disable IOMMU SNP enforcement with SNP_SHUTDOWN_EX command before the kdump kernel starts booting as we can't
setup IOMMU CWB again in kdump as SEV-SNP exclusion base and range limit registers are locked as IOMMU SNP support is still enabled.
I tried to use the previous kernel's CWB (cmd_sem) as below:
static int __init alloc_cwwb_sem(struct amd_iommu *iommu)
{
if (!is_kdump_kernel())
iommu->cmd_sem = iommu_alloc_4k_pages(iommu, GFP_KERNEL, 1);
else {
if (check_feature(FEATURE_SNP)) {
u64 cwwb_sem_paddr;
cwwb_sem_paddr = readq(iommu->mmio_base + MMIO_EXCL_BASE_OFFSET);
iommu->cmd_sem = iommu_phys_to_virt(cwwb_sem_paddr);
return iommu->cmd_sem ? 0 : -ENOMEM;
}
}
return iommu->cmd_sem ? 0 : -ENOMEM;
}
I tried this, but this fails as i believe the kdump kernel will not have these previous kernel's allocated IOMMU CWB in the kernel
direct map :
[ 1.708959] AMD-Vi: in alloc_cwwb_sem kdump kernel
[ 1.714327] AMD-Vi: in alloc_cwwb_sem SNP feature enabled, cmd_sem_paddr 0x100805000, cmd_sem_vaddr 0xffff9f5340805000
[ 1.726309] AMD-Vi: in alloc_cwwb_sem kdump kernel
[ 1.731676] AMD-Vi: in alloc_cwwb_sem SNP feature enabled, cmd_sem_paddr 0x1050051000, cmd_sem_vaddr 0xffff9f6290051000
[ 1.743742] AMD-Vi: in alloc_cwwb_sem kdump kernel
[ 1.749109] AMD-Vi: in alloc_cwwb_sem SNP feature enabled, cmd_sem_paddr 0x1050052000, cmd_sem_vaddr 0xffff9f6290052000
[ 1.761177] AMD-Vi: in alloc_cwwb_sem kdump kernel
[ 1.766542] AMD-Vi: in alloc_cwwb_sem SNP feature enabled, cmd_sem_paddr 0x100808000, cmd_sem_vaddr 0xffff9f5340808000
[ 1.778509] AMD-Vi: in alloc_cwwb_sem kdump kernel
[ 1.783877] AMD-Vi: in alloc_cwwb_sem SNP feature enabled, cmd_sem_paddr 0x1050053000, cmd_sem_vaddr 0xffff9f6290053000
[ 1.795942] AMD-Vi: in alloc_cwwb_sem kdump kernel
[ 1.801300] AMD-Vi: in alloc_cwwb_sem SNP feature enabled, cmd_sem_paddr 0x100809000, cmd_sem_vaddr 0xffff9f5340809000
[ 1.813268] AMD-Vi: in alloc_cwwb_sem kdump kernel
[ 1.818636] AMD-Vi: in alloc_cwwb_sem SNP feature enabled, cmd_sem_paddr 0x1050054000, cmd_sem_vaddr 0xffff9f6290054000
[ 1.830701] AMD-Vi: in alloc_cwwb_sem kdump kernel
[ 1.836069] AMD-Vi: in alloc_cwwb_sem SNP feature enabled, cmd_sem_paddr 0x10080a000, cmd_sem_vaddr 0xffff9f534080a000
[ 1.848039] AMD-Vi: early_amd_iommu_init: irq remaping enabled
[ 1.855431] AMD-Vi: in early_enable_iommus
[ 1.860032] AMD-Vi: Translation is already enabled - trying to copy translation structures
[ 1.869812] AMD-Vi: Copied DEV table from previous kernel.
[ 1.875958] AMD-Vi: in build_completion_wait, paddr = 0x100805000
[ 1.882766] BUG: unable to handle page fault for address: ffff9f5340805000
[ 1.890441] #PF: supervisor read access in kernel mode
[ 1.896177] #PF: error_code(0x0000) - not-present page
....
I think that memremap(..,..,MEMREMAP_WB) will also fail for the same reason as memremap(.., MEMREMAP_WB) for the RAM region will
again use the kernel directmap.
So it looks like we need to support IOMMU_SNP_SHUTDOWN with SNP_SHUTDOWN_EX command before kdump kernel starts booting.
Thanks,
Ashish
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