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Subject[tip: ras/core] x86/mce: Remove noinstr annotation from mce_setup()
The following commit has been merged into the ras/core branch of tip:

Commit-ID: 487d654db3edacc31dee86b10258cc740640fad8
Gitweb: https://git.kernel.org/tip/487d654db3edacc31dee86b10258cc740640fad8
Author: Borislav Petkov <bp@suse.de>
AuthorDate: Tue, 05 Oct 2021 19:54:47 +02:00
Committer: Borislav Petkov <bp@suse.de>
CommitterDate: Mon, 13 Dec 2021 14:12:21 +01:00

x86/mce: Remove noinstr annotation from mce_setup()

Instead, sandwitch around the call which is done in noinstr context and
mark the caller - mce_gather_info() - as noinstr.

Also, document what the whole instrumentation strategy with #MC is going
to be in the future and where it all is supposed to be going to.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20211208111343.8130-5-bp@alien8.de
---
arch/x86/kernel/cpu/mce/core.c | 26 ++++++++++++++++++++------
1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
index 87a277f..c11fc8d 100644
--- a/arch/x86/kernel/cpu/mce/core.c
+++ b/arch/x86/kernel/cpu/mce/core.c
@@ -127,7 +127,7 @@ static struct irq_work mce_irq_work;
BLOCKING_NOTIFIER_HEAD(x86_mce_decoder_chain);

/* Do initial initialization of a struct mce */
-noinstr void mce_setup(struct mce *m)
+void mce_setup(struct mce *m)
{
memset(m, 0, sizeof(struct mce));
m->cpu = m->extcpu = smp_processor_id();
@@ -430,9 +430,15 @@ static noinstr void mce_wrmsrl(u32 msr, u64 v)
* check into our "mce" struct so that we can use it later to assess
* the severity of the problem as we read per-bank specific details.
*/
-static inline void mce_gather_info(struct mce *m, struct pt_regs *regs)
+static noinstr void mce_gather_info(struct mce *m, struct pt_regs *regs)
{
+ /*
+ * Enable instrumentation around mce_setup() which calls external
+ * facilities.
+ */
+ instrumentation_begin();
mce_setup(m);
+ instrumentation_end();

m->mcgstatus = mce_rdmsrl(MSR_IA32_MCG_STATUS);
if (regs) {
@@ -1312,11 +1318,11 @@ static noinstr void unexpected_machine_check(struct pt_regs *regs)
}

/*
- * The actual machine check handler. This only handles real
- * exceptions when something got corrupted coming in through int 18.
+ * The actual machine check handler. This only handles real exceptions when
+ * something got corrupted coming in through int 18.
*
- * This is executed in NMI context not subject to normal locking rules. This
- * implies that most kernel services cannot be safely used. Don't even
+ * This is executed in #MC context not subject to normal locking rules.
+ * This implies that most kernel services cannot be safely used. Don't even
* think about putting a printk in there!
*
* On Intel systems this is entered on all CPUs in parallel through
@@ -1328,6 +1334,14 @@ static noinstr void unexpected_machine_check(struct pt_regs *regs)
* issues: if the machine check was due to a failure of the memory
* backing the user stack, tracing that reads the user stack will cause
* potentially infinite recursion.
+ *
+ * Currently, the #MC handler calls out to a number of external facilities
+ * and, therefore, allows instrumentation around them. The optimal thing to
+ * have would be to do the absolutely minimal work required in #MC context
+ * and have instrumentation disabled only around that. Further processing can
+ * then happen in process context where instrumentation is allowed. Achieving
+ * that requires careful auditing and modifications. Until then, the code
+ * allows instrumentation temporarily, where required. *
*/
noinstr void do_machine_check(struct pt_regs *regs)
{
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