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Message-ID: <9017076fe1050b8a38d5ee8447b827a5c41c4e72.1660579062.git.thomas.lendacky@amd.com>
Date: Mon, 15 Aug 2022 10:57:42 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: <linux-kernel@...r.kernel.org>, <x86@...nel.org>
CC: Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"Kirill A. Shutemov" <kirill@...temov.name>,
"H. Peter Anvin" <hpa@...or.com>,
Michael Roth <michael.roth@....com>,
Joerg Roedel <jroedel@...e.de>,
Andy Lutomirski <luto@...nel.org>,
Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH v3 1/2] x86/sev: Put PSC struct on the stack in prep for unaccepted memory support
In advance of providing support for unaccepted memory, switch from using
kmalloc() for allocating the Page State Change (PSC) structure to using a
local variable that lives on the stack. This is needed to avoid a possible
recursive call into set_pages_state() if the kmalloc() call requires
(more) memory to be accepted, which would result in a hang.
The current size of the PSC struct is 2,032 bytes. To make the struct more
stack friendly, reduce the number of PSC entries from 253 down to 64,
resulting in a size of 520 bytes. This is a nice compromise on struct size
and total PSC requests while still allowing parallel PSC operations across
vCPUs.
Also, since set_pages_state() uses the per-CPU GHCB, add a static variable
that indicates when per-CPU GHCBs are available. Until they are available,
the GHCB MSR protocol is used to perform page state changes. For APs, the
per-CPU GHCB is created before they are started and registered upon
startup, so this flag can be used globally for the BSP and APs instead of
creating a per-CPU flag.
If either the reduction in PSC entries or the use of the MSR protocol
until the per-CPU GHCBs are available, result in any kind of performance
issue (that is not seen at the moment), use of a larger static PSC struct,
with fallback to the smaller stack version, or use the boot GHCB prior to
per-CPU GHCBs, can be investigated.
Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
---
arch/x86/include/asm/sev-common.h | 9 +++++++--
arch/x86/kernel/sev.c | 32 +++++++++++++++++++++++--------
2 files changed, 31 insertions(+), 10 deletions(-)
diff --git a/arch/x86/include/asm/sev-common.h b/arch/x86/include/asm/sev-common.h
index b8357d6ecd47..6c3d61c5f6a3 100644
--- a/arch/x86/include/asm/sev-common.h
+++ b/arch/x86/include/asm/sev-common.h
@@ -106,8 +106,13 @@ enum psc_op {
#define GHCB_HV_FT_SNP BIT_ULL(0)
#define GHCB_HV_FT_SNP_AP_CREATION BIT_ULL(1)
-/* SNP Page State Change NAE event */
-#define VMGEXIT_PSC_MAX_ENTRY 253
+/*
+ * SNP Page State Change NAE event
+ * The VMGEXIT_PSC_MAX_ENTRY determines the size of the PSC structure,
+ * which is a local variable (stack usage) in set_pages_state(). Do not
+ * increase this value without evaluating the impact to stack usage.
+ */
+#define VMGEXIT_PSC_MAX_ENTRY 64
struct psc_hdr {
u16 cur_entry;
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index c05f0124c410..40268ce97aad 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -66,6 +66,17 @@ static struct ghcb boot_ghcb_page __bss_decrypted __aligned(PAGE_SIZE);
*/
static struct ghcb *boot_ghcb __section(".data");
+/*
+ * A flag used by set_pages_state() that indicates when the per-CPU GHCB has
+ * been created and registered and thus can be used instead of using the MSR
+ * protocol. The set_pages_state() function eventually invokes vmgexit_psc(),
+ * which only works with a per-CPU GHCB.
+ *
+ * For APs, the per-CPU GHCB is created before they are started and registered
+ * upon startup, so this flag can be used globally for the BSP and APs.
+ */
+static bool ghcb_percpu_ready __section(".data");
+
/* Bitmap of SEV features supported by the hypervisor */
static u64 sev_hv_features __ro_after_init;
@@ -660,7 +671,7 @@ static void pvalidate_pages(unsigned long vaddr, unsigned int npages, bool valid
}
}
-static void __init early_set_pages_state(unsigned long paddr, unsigned int npages, enum psc_op op)
+static void early_set_pages_state(unsigned long paddr, unsigned int npages, enum psc_op op)
{
unsigned long paddr_end;
u64 val;
@@ -751,6 +762,8 @@ static int vmgexit_psc(struct snp_psc_desc *desc)
unsigned long flags;
struct ghcb *ghcb;
+ WARN_ON_ONCE(!ghcb_percpu_ready);
+
/*
* __sev_get_ghcb() needs to run with IRQs disabled because it is using
* a per-CPU GHCB.
@@ -868,11 +881,14 @@ static void __set_pages_state(struct snp_psc_desc *data, unsigned long vaddr,
static void set_pages_state(unsigned long vaddr, unsigned int npages, int op)
{
unsigned long vaddr_end, next_vaddr;
- struct snp_psc_desc *desc;
+ struct snp_psc_desc desc = {};
- desc = kmalloc(sizeof(*desc), GFP_KERNEL_ACCOUNT);
- if (!desc)
- panic("SNP: failed to allocate memory for PSC descriptor\n");
+ /*
+ * Use the MSR protocol when the per-CPU GHCBs are not yet registered,
+ * since vmgexit_psc() uses the per-CPU GHCB.
+ */
+ if (!ghcb_percpu_ready)
+ return early_set_pages_state(__pa(vaddr), npages, op);
vaddr = vaddr & PAGE_MASK;
vaddr_end = vaddr + (npages << PAGE_SHIFT);
@@ -882,12 +898,10 @@ static void set_pages_state(unsigned long vaddr, unsigned int npages, int op)
next_vaddr = min_t(unsigned long, vaddr_end,
(VMGEXIT_PSC_MAX_ENTRY * PAGE_SIZE) + vaddr);
- __set_pages_state(desc, vaddr, next_vaddr, op);
+ __set_pages_state(&desc, vaddr, next_vaddr, op);
vaddr = next_vaddr;
}
-
- kfree(desc);
}
void snp_set_memory_shared(unsigned long vaddr, unsigned int npages)
@@ -1254,6 +1268,8 @@ void setup_ghcb(void)
if (cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
snp_register_per_cpu_ghcb();
+ ghcb_percpu_ready = true;
+
return;
}
--
2.36.1
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