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[PATCH 1/2] x86/time: use fake read_tsc()
- To: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 1 Mar 2022 12:05:47 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Tue, 01 Mar 2022 11:05:57 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
Go a step further than bed9ae54df44 ("x86/time: switch platform timer
hooks to altcall") did and eliminate the "real" read_tsc() altogether:
It's not used except in pointer comparisons, and hence it looks overall
more safe to simply poison plt_tsc's read_counter hook.
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
I wasn't really sure whether it would be better to use simply void * for
the type of the expression, resulting in an undesirable data -> function
pointer conversion, but making it impossible to mistakenly try and call
the (fake) function directly.
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -585,10 +585,12 @@ static s64 __init cf_check init_tsc(stru
return ret;
}
-static uint64_t __init cf_check read_tsc(void)
-{
- return rdtsc_ordered();
-}
+/*
+ * plt_tsc's read_counter hook is (and should not be) invoked via the struct
+ * field. To avoid carrying an unused, indirectly reachable function, poison
+ * the field with an easily identifiable non-canonical pointer.
+ */
+#define read_tsc ((uint64_t(*)(void))0x75C75C75C75C75C0ul)
static struct platform_timesource __initdata plt_tsc =
{
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