linux-kernel - [PATCH v4 2/4] x86/sev: Allow for use of the early boot GHCB for PSC requests

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Date:   Thu, 25 Aug 2022 09:23:15 -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 v4 2/4] x86/sev: Allow for use of the early boot GHCB for PSC requests

Using a GHCB for a page stage change (as opposed to the MSR protocol)
allows for multiple pages to be processed in a single request. In prep
for early PSC requests in support of unaccepted memory, update the
invocation of vmgexit_psc() to be able to use the early boot GHCB and not
just the per-CPU GHCB structure.

In order to use the proper GHCB (early boot vs per-CPU), set a flag that
indicates when the per-CPU GHCBs are available and registered. For APs,
the per-CPU GHCBs are 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. This will allow for a significant reduction in
the number of MSR protocol page state change requests when accepting
memory.

Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
---
 arch/x86/kernel/sev.c | 61 +++++++++++++++++++++++++++----------------
 1 file changed, 38 insertions(+), 23 deletions(-)

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index d18a580dd048..a5f02b6b099b 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -117,7 +117,19 @@ static DEFINE_PER_CPU(struct sev_es_save_area *, sev_vmsa);
 
 struct sev_config {
 	__u64 debug		: 1,
-	      __reserved	: 63;
+
+	      /*
+	       * A flag used by __set_pages_state() that indicates when the
+	       * per-CPU GHCB has been created and registered and thus can be
+	       * used by the BSP instead of the early boot 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.
+	       */
+	      ghcbs_initialized	: 1,
+
+	      __reserved	: 62;
 };
 
 static struct sev_config sev_cfg __read_mostly;
@@ -660,7 +672,7 @@ 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;
@@ -742,26 +754,13 @@ void __init snp_prep_memory(unsigned long paddr, unsigned int sz, enum psc_op op
 		WARN(1, "invalid memory op %d\n", op);
 }
 
-static int vmgexit_psc(struct snp_psc_desc *desc)
+static int vmgexit_psc(struct ghcb *ghcb, struct snp_psc_desc *desc)
 {
 	int cur_entry, end_entry, ret = 0;
 	struct snp_psc_desc *data;
-	struct ghcb_state state;
 	struct es_em_ctxt ctxt;
-	unsigned long flags;
-	struct ghcb *ghcb;
 
-	/*
-	 * __sev_get_ghcb() needs to run with IRQs disabled because it is using
-	 * a per-CPU GHCB.
-	 */
-	local_irq_save(flags);
-
-	ghcb = __sev_get_ghcb(&state);
-	if (!ghcb) {
-		ret = 1;
-		goto out_unlock;
-	}
+	vc_ghcb_invalidate(ghcb);
 
 	/* Copy the input desc into GHCB shared buffer */
 	data = (struct snp_psc_desc *)ghcb->shared_buffer;
@@ -818,20 +817,18 @@ static int vmgexit_psc(struct snp_psc_desc *desc)
 	}
 
 out:
-	__sev_put_ghcb(&state);
-
-out_unlock:
-	local_irq_restore(flags);
-
 	return ret;
 }
 
 static void __set_pages_state(struct snp_psc_desc *data, unsigned long vaddr,
 			      unsigned long vaddr_end, int op)
 {
+	struct ghcb_state state;
 	struct psc_hdr *hdr;
 	struct psc_entry *e;
+	unsigned long flags;
 	unsigned long pfn;
+	struct ghcb *ghcb;
 	int i;
 
 	hdr = &data->hdr;
@@ -861,8 +858,20 @@ static void __set_pages_state(struct snp_psc_desc *data, unsigned long vaddr,
 		i++;
 	}
 
-	if (vmgexit_psc(data))
+	local_irq_save(flags);
+
+	if (sev_cfg.ghcbs_initialized)
+		ghcb = __sev_get_ghcb(&state);
+	else
+		ghcb = boot_ghcb;
+
+	if (!ghcb || vmgexit_psc(ghcb, data))
 		sev_es_terminate(SEV_TERM_SET_LINUX, GHCB_TERM_PSC);
+
+	if (sev_cfg.ghcbs_initialized)
+		__sev_put_ghcb(&state);
+
+	local_irq_restore(flags);
 }
 
 static void set_pages_state(unsigned long vaddr, unsigned int npages, int op)
@@ -870,6 +879,10 @@ static void set_pages_state(unsigned long vaddr, unsigned int npages, int op)
 	unsigned long vaddr_end, next_vaddr;
 	struct snp_psc_desc desc;
 
+	/* Use the MSR protocol when a GHCB is not available. */
+	if (!boot_ghcb)
+		return early_set_pages_state(__pa(vaddr), npages, op);
+
 	vaddr = vaddr & PAGE_MASK;
 	vaddr_end = vaddr + (npages << PAGE_SHIFT);
 
@@ -1248,6 +1261,8 @@ void setup_ghcb(void)
 		if (cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
 			snp_register_per_cpu_ghcb();
 
+		sev_cfg.ghcbs_initialized = true;
+
 		return;
 	}
 
-- 
2.37.2