linux-kernel - Re: [PATCH V8 30/44] mm/pkeys: Test setting a PKS key in a custom fault callback

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Date:   Tue, 1 Feb 2022 00:55:47 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "hpa@...or.com" <hpa@...or.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "Weiny, Ira" <ira.weiny@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
CC:     "Yu, Fenghua" <fenghua.yu@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V8 30/44] mm/pkeys: Test setting a PKS key in a custom
 fault callback

On Thu, 2022-01-27 at 09:54 -0800, ira.weiny@...el.com wrote:
> Add a test which does this.
> 
>         $ echo 5 > /sys/kernel/debug/x86/run_pks
>         $ cat /sys/kernel/debug/x86/run_pks
>         PASS

Hmm, when I run this on qemu TCG, I get:

root@(none):/# echo 5 > /sys/kernel/debug/x86/run_pks
[   29.438159] pks_test: Failed to see the callback
root@(none):/# cat /sys/kernel/debug/x86/run_pks
FAIL

I think it's a problem with the test though. The generated code is not
expecting fault_callback_ctx.callback_seen to get changed in the
exception. The following fixed it for me:

diff --git a/lib/pks/pks_test.c b/lib/pks/pks_test.c
index 1528df0bb283..d979d2afe921 100644
--- a/lib/pks/pks_test.c
+++ b/lib/pks/pks_test.c
@@ -570,6 +570,7 @@ static bool run_fault_clear_test(void)
        /* fault */
        memcpy(test_page, ctx->data, 8);
 
+       barrier();
        if (!fault_callback_ctx.callback_seen) {
                pr_err("Failed to see the callback\n");
                rc = false;

But, I wonder if volatile is also needed on the read to be fully
correct. I usually have to consult the docs when I deal with that
stuff...