Re: [PATCH 1/4] iommu/amd: Check SNP support before enabling IOMMU

Sean Christopherson
2025-01-25T00:39:39Z

On Fri, Jan 24, 2025, Ashish Kalra wrote:
> With discussions with the AMD IOMMU team, here is the AMD IOMMU
> initialization flow:

..

> IOMMU SNP check
> Core IOMMU subsystem init is done during iommu_subsys_init() via
> subsys_initcall. This function does change the DMA mode depending on
> kernel config. Hence, SNP check should be done after subsys_initcall.
> That's why its done currently during IOMMU PCI init (IOMMU_PCI_INIT stage).
> And for that reason snp_rmptable_init() is currently invoked via
> device_initcall().
>
> The summary is that we cannot move snp_rmptable_init() to subsys_initcall as
> core IOMMU subsystem gets initialized via subsys_initcall.

Just explicitly invoke RMP initialization during IOMMU SNP setup. Pretending
there's no connection when snp_rmptable_init() checks amd_iommu_snp_en and has
a comment saying it needs to come after IOMMU SNP setup is ridiculous.

Compile tested only.

---
From: Sean Christopherson <seanjc@google.com>
Date: Fri, 24 Jan 2025 16:25:58 -0800
Subject: [PATCH] x86/sev: iommu/amd: Explicitly init SNP's RMP table during
IOMMU SNP setup

Explicitly initialize the RMP table during IOMMU SNP setup, as there is a
hard dependency on the IOMMU being configured first, and dancing around
the dependency with initcall shenanigans and a comment is all kinds of
stupid.

The RMP is blatantly not a device; initializing it via a device_initcall()
is confusing and "works" only because of dumb luck: due to kernel build
order, when the the PSP driver is built-in, its effective device_initcall()
just so happens to be invoked after snp_rmptable_init().

That all falls apart if the order is changed in any way. E.g. if KVM
is built-in and attempts to access the RMP during its device_initcall(),
chaos ensues.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/include/asm/sev.h | 1 +
arch/x86/virt/svm/sev.c | 25 ++++++++-----------------
drivers/iommu/amd/init.c | 7 ++++++-
3 files changed, 15 insertions(+), 18 deletions(-)

diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index 91f08af31078..30da0fc15923 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -503,6 +503,7 @@ static inline void snp_kexec_begin(void) { }

#ifdef CONFIG_KVM_AMD_SEV
bool snp_probe_rmptable_info(void);
+int __init snp_rmptable_init(void);
int snp_lookup_rmpentry(u64 pfn, bool *assigned, int *level);
void snp_dump_hva_rmpentry(unsigned long address);
int psmash(u64 pfn);
diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
index 9a6a943d8e41..d932aa21340b 100644
--- a/arch/x86/virt/svm/sev.c
+++ b/arch/x86/virt/svm/sev.c
@@ -189,19 +189,19 @@ void __init snp_fixup_e820_tables(void)
* described in the SNP_INIT_EX firmware command description in the SNP
* firmware ABI spec.
*/
-static int __init snp_rmptable_init(void)
+int __init snp_rmptable_init(void)
{
u64 max_rmp_pfn, calc_rmp_sz, rmptable_size, rmp_end, val;
void *rmptable_start;

- if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))
- return 0;
+ if (WARN_ON_ONCE(!cc_platform_has(CC_ATTR_HOST_SEV_SNP)))
+ return -ENOSYS;

- if (!amd_iommu_snp_en)
- goto nosnp;
+ if (WARN_ON_ONCE(!amd_iommu_snp_en))
+ return -ENOSYS;

if (!probed_rmp_size)
- goto nosnp;
+ return -ENOSYS;

rmp_end = probed_rmp_base + probed_rmp_size - 1;

@@ -218,13 +218,13 @@ static int __init snp_rmptable_init(void)
if (calc_rmp_sz > probed_rmp_size) {
pr_err("Memory reserved for the RMP table does not cover full system RAM (expected 0x%llx got 0x%llx)\n",
calc_rmp_sz, probed_rmp_size);
- goto nosnp;
+ return -ENOSYS;
}

rmptable_start = memremap(probed_rmp_base, probed_rmp_size, MEMREMAP_WB);
if (!rmptable_start) {
pr_err("Failed to map RMP table\n");
- goto nosnp;
+ return -ENOMEM;
}

/*
@@ -261,17 +261,8 @@ static int __init snp_rmptable_init(void)
crash_kexec_post_notifiers = true;

return 0;
-
-nosnp:
- cc_platform_clear(CC_ATTR_HOST_SEV_SNP);
- return -ENOSYS;
}

-/*
- * This must be called after the IOMMU has been initialized.
- */
-device_initcall(snp_rmptable_init);
-
static struct rmpentry *get_rmpentry(u64 pfn)
{
if (WARN_ON_ONCE(pfn > rmptable_max_pfn))
diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
index 0e0a531042ac..d00530156a72 100644
--- a/drivers/iommu/amd/init.c
+++ b/drivers/iommu/amd/init.c
@@ -3171,7 +3171,7 @@ static bool __init detect_ivrs(void)
return true;
}

-static void iommu_snp_enable(void)
+static __init void iommu_snp_enable(void)
{
#ifdef CONFIG_KVM_AMD_SEV
if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))
@@ -3196,6 +3196,11 @@ static void iommu_snp_enable(void)
goto disable_snp;
}

+ if (snp_rmptable_init()) {
+ pr_warn("SNP: RMP initialization failed, SNP cannot be supported.\n");
+ goto disable_snp;
+ }
+
pr_info("IOMMU SNP support enabled.\n");
return;

base-commit: ac80076177131f6e3291737c851a6fe32cc03fd3
--