On 29/09/16 14:53, Igor Druzhinin wrote:
> As long as t_info_first_offset is calculated in uint32_t offsets we need to
> multiply it by sizeof(uint32_t) in order to get the right number of pages
> for trace metadata. Not doing that makes it impossible to read the trace
> buffer correctly from userspace for some corner cases.
>
> Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
Hmm, looks like we actually want to revert this c/s fbf96e6, "xentrace:
correct formula to calculate t_info_pages". But that one was presumably
written (and Acked by me) because the variable name there,
t_info_first_offset, is confusing.
The other mistake in fbf96e6 is that before t_info_words was actually
denominated in words; but after it's denominated in bytes (which is
again confusing).
What about something like the attached instead? This should fix your
problem while making the code clearer.
-George
From b0252cec4a96fea28faa85c47ab0f5d5997444ad Mon Sep 17 00:00:00 2001
From: George Dunlap <george.dunlap@xxxxxxxxxx>
Date: Fri, 30 Sep 2016 15:42:56 +0100
Subject: [PATCH] xen/trace: Fix trace metadata page count calculation (revert
fbf96e6)
Changeset fbf96e6, "xentrace: correct formula to calculate
t_info_pages", broke the trace metadata page count calculation, by
mistaking t_info_first_offset as denominated in bytes, when in fact it
is denominated in words (uint32_t).
Effectively revert that change, and put a comment there to reduce the
chance that someone will make that mistake in the future.
Spotted-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
---
CC: Olaf Hering <olaf@xxxxxxxxx>
CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Jan Beulich <jbeulich@xxxxxxxx>
---
xen/common/trace.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/xen/common/trace.c b/xen/common/trace.c
index f651cf3..2f4ecca 100644
--- a/xen/common/trace.c
+++ b/xen/common/trace.c
@@ -148,8 +148,12 @@ static int calculate_tbuf_size(unsigned int pages, uint16_t t_info_first_offset)
pages = max_pages;
}
- t_info_words = num_online_cpus() * pages * sizeof(uint32_t);
- t_info_pages = PFN_UP(t_info_first_offset + t_info_words);
+ /*
+ * NB this calculation is correct, because t_info_first_offset is
+ * in words, not bytes, not bytes
