From 194125e3ebd553acb02aaf3797a4f0387493fe94 Mon Sep 17 00:00:00 2001 From: "Emilio G. Cota" Date: Wed, 2 Aug 2017 20:34:06 -0400 Subject: translate-all: protect TB jumps with a per-destination-TB lock MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit This applies to both user-mode and !user-mode emulation. Instead of relying on a global lock, protect the list of incoming jumps with tb->jmp_lock. This lock also protects tb->cflags, so update all tb->cflags readers outside tb->jmp_lock to use atomic reads via tb_cflags(). In order to find the destination TB (and therefore its jmp_lock) from the origin TB, we introduce tb->jmp_dest[]. I considered not using a linked list of jumps, which simplifies code and makes the struct smaller. However, it unnecessarily increases memory usage, which results in a performance decrease. See for instance these numbers booting+shutting down debian-arm: Time (s) Rel. err (%) Abs. err (s) Rel. slowdown (%) ------------------------------------------------------------------------------ before 20.88 0.74 0.154512 0. after 20.81 0.38 0.079078 -0.33524904 GTree 21.02 0.28 0.058856 0.67049808 GHashTable + xxhash 21.63 1.08 0.233604 3.5919540 Using a hash table or a binary tree to keep track of the jumps doesn't really pay off, not only due to the increased memory usage, but also because most TBs have only 0 or 1 jumps to them. The maximum number of jumps when booting debian-arm that I measured is 35, but as we can see in the histogram below a TB with that many incoming jumps is extremely rare; the average TB has 0.80 incoming jumps. n_jumps: 379208; avg jumps/tb: 0.801099 dist: [0.0,1.0)|▄█▁▁▁▁▁▁▁▁▁▁▁ ▁▁▁▁▁▁ ▁▁▁ ▁▁▁ ▁|[34.0,35.0] Reviewed-by: Richard Henderson Signed-off-by: Emilio G. Cota Signed-off-by: Richard Henderson --- docs/devel/multi-thread-tcg.txt | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) (limited to 'docs') diff --git a/docs/devel/multi-thread-tcg.txt b/docs/devel/multi-thread-tcg.txt index faf09c6..df83445 100644 --- a/docs/devel/multi-thread-tcg.txt +++ b/docs/devel/multi-thread-tcg.txt @@ -131,8 +131,10 @@ DESIGN REQUIREMENT: Safely handle invalidation of TBs The direct jump themselves are updated atomically by the TCG tb_set_jmp_target() code. Modification to the linked lists that allow -searching for linked pages are done under the protect of the -tb_lock(). +searching for linked pages are done under the protection of tb->jmp_lock, +where tb is the destination block of a jump. Each origin block keeps a +pointer to its destinations so that the appropriate lock can be acquired before +iterating over a jump list. The global page table is a lockless radix tree; cmpxchg is used to atomically insert new elements. -- cgit v1.1