On 6/10/2022 6:18 am, Sean Christopherson wrote:
On Fri, Aug 19, 2022, Like Xu wrote:
From: Like Xu <likexu@xxxxxxxxxxx>
The pmu test check_counter_overflow() always fails with the "./configure
--arch=i386".
Explicitly state that the failures are with 32-bit binaries. E.g. I can and do
run KUT in 32-bit VMs, which doesn't require the explicit --arch=i386.
True and applied.
The cnt.count obtained from the latter run of measure()
(based on fixed counter 0) is not equal to the expected value (based
on gp counter 0) and there is a positive error with a value of 2.
The two extra instructions come from inline wrmsr() and inline rdmsr()
inside the global_disable() binary code block. Specifically, for each msr
access, the i386 code will have two assembly mov instructions before
rdmsr/wrmsr (mark it for fixed counter 0, bit 32), but only one assembly
mov is needed for x86_64 and gp counter 0 on i386.
Fix the expected init cnt.count for fixed counter 0 overflow based on
the same fixed counter 0, not always using gp counter 0.
You lost me here. I totally understand the problem, but I don't understand the
fix.
The sequence of instructions to count events using the #GP and #Fixed counters
is different.
Thus the fix is quite high level, to use the same counter (w/ same instruction
sequences) to
set initial value for the same counter.
I may add this to the commit message.
Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
---
x86/pmu.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/x86/pmu.c b/x86/pmu.c
index 45ca2c6..057fd4a 100644
--- a/x86/pmu.c
+++ b/x86/pmu.c
@@ -315,6 +315,9 @@ static void check_counter_overflow(void)
if (i == nr_gp_counters) {
cnt.ctr = fixed_events[0].unit_sel;
+ __measure(&cnt, 0);
+ count = cnt.count;
+ cnt.count = 1 - count;
This definitely needs a comment.
Dumb question time: if the count is off by 2, why can't we just subtract 2?
More low-level code (bringing in differences between the 32-bit and 64-bit runtimes)
being added would break this.
The test goal is simply to set the initial value of a counter to overflow, which
is always
off by 1, regardless of the involved rd/wrmsr or other execution details.
#ifndef __x86_64__
/* comment about extra MOV insns for RDMSR/WRMSR */
cnt.count -= 2;
#endif
cnt.count &= (1ull << pmu_fixed_counter_width()) - 1;
}
--
2.37.2
