[PATCH 3/3] x86/sev-es: Improve comments in and around __sev_es_ist_enter/exit()

From: Joerg Roedel <joro@8bytes.org>
To: x86@kernel.org
Cc: kvm@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	virtualization@lists.linux-foundation.org,
	Arvind Sankar <nivedita@alum.mit.edu>,
	hpa@zytor.com, Jiri Slaby <jslaby@suse.cz>,
	Joerg Roedel <joro@8bytes.org>,
	David Rientjes <rientjes@google.com>,
	Martin Radev <martin.b.radev@gmail.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Joerg Roedel <jroedel@suse.de>, Kees Cook <keescook@chromium.org>,
	Cfir Cohen <cfir@google.com>, Andy Lutomirski <luto@kernel.org>,
	Dan Williams <dan.j.williams@intel.com>,
	Juergen Gross <jgross@suse.com>, Mike Stunes <mstunes@vmware.com>,
	Sean Christopherson <seanjc@google.com>,
	linux-kernel@vger.kernel.org,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Erdem Aktas <erdemaktas@google.com>
Subject: [PATCH 3/3] x86/sev-es: Improve comments in and around __sev_es_ist_enter/exit()
Date: Wed, 17 Feb 2021 13:01:43 +0100	[thread overview]
Message-ID: <20210217120143.6106-4-joro@8bytes.org> (raw)
In-Reply-To: <20210217120143.6106-1-joro@8bytes.org>

From: Joerg Roedel <jroedel@suse.de>

Better explain why this code is necessary and what it is doing.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
 arch/x86/kernel/sev-es.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
index 0df38b185d53..79241bc45f25 100644
--- a/arch/x86/kernel/sev-es.c
+++ b/arch/x86/kernel/sev-es.c
@@ -127,14 +127,20 @@ static __always_inline bool on_vc_stack(unsigned long sp)
 }
 
 /*
- * This function handles the case when an NMI is raised in the #VC exception
- * handler entry code. In this case, the IST entry for #VC must be adjusted, so
- * that any subsequent #VC exception will not overwrite the stack contents of the
- * interrupted #VC handler.
+ * This function handles the case when an NMI is raised in the #VC
+ * exception handler entry code, before the #VC handler has switched off
+ * its IST stack. In this case, the IST entry for #VC must be adjusted,
+ * so that any nested #VC exception will not overwrite the stack
+ * contents of the interrupted #VC handler.
  *
  * The IST entry is adjusted unconditionally so that it can be also be
- * unconditionally adjusted back in sev_es_ist_exit(). Otherwise a nested
- * sev_es_ist_exit() call may adjust back the IST entry too early.
+ * unconditionally adjusted back in __sev_es_ist_exit(). Otherwise a
+ * nested sev_es_ist_exit() call may adjust back the IST entry too
+ * early.
+ *
+ * The __sev_es_ist_enter() and __sev_es_ist_exit() functions always run
+ * on the NMI IST stack, as they are only called from NMI handling code
+ * right now.
  */
 void noinstr __sev_es_ist_enter(struct pt_regs *regs)
 {
@@ -143,7 +149,10 @@ void noinstr __sev_es_ist_enter(struct pt_regs *regs)
 	/* Read old IST entry */
 	old_ist = __this_cpu_read(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC]);
 
-	/* Make room on the IST stack */
+	/*
+	 * Make room on the IST stack - Reserve 8 bytes to store the old
+	 * IST entry.
+	 */
 	if (on_vc_stack(regs->sp) &&
 	    !user_mode(regs) &&
 	    !from_syscall_gap(regs))
-- 
2.30.0

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