[qemu-xen staging] tcg: update comments for save_iotlb_data in cputlb

[patchbot@xxxxxxx]

commit 570ef3093b5ed327249397ad0295cf01c67d9db4
Author:     Alex Bennée <alex.bennee@xxxxxxxxxx>
AuthorDate: Mon Jul 20 13:23:58 2020 +0100
Commit:     Richard Henderson <richard.henderson@xxxxxxxxxx>
CommitDate: Fri Jul 24 14:25:11 2020 -0700

    tcg: update comments for save_iotlb_data in cputlb
    
    I missed Emilio's review comments:
    
      Message-ID: <20200718205107.GA994221@sff>
    
    and the patch got merged. Correcting the comments now.
    
    Reviewed-by: Emilio G. Cota <cota@xxxxxxxxx>
    Signed-off-by: Alex Bennée <alex.bennee@xxxxxxxxxx>
    Message-Id: <20200720122358.26881-1-alex.bennee@xxxxxxxxxx>
    Signed-off-by: Richard Henderson <richard.henderson@xxxxxxxxxx>
---
 accel/tcg/cputlb.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/accel/tcg/cputlb.c b/accel/tcg/cputlb.c
index d370aedb47..5698292749 100644
--- a/accel/tcg/cputlb.c
+++ b/accel/tcg/cputlb.c
@@ -1075,10 +1075,8 @@ static uint64_t io_readx(CPUArchState *env, 
CPUIOTLBEntry *iotlbentry,
 
 /*
  * Save a potentially trashed IOTLB entry for later lookup by plugin.
- *
- * We also need to track the thread storage address because the RCU
- * cleanup that runs when we leave the critical region (the current
- * execution) is actually in a different thread.
+ * This is read by tlb_plugin_lookup if the iotlb entry doesn't match
+ * because of the side effect of io_writex changing memory layout.
  */
 static void save_iotlb_data(CPUState *cs, hwaddr addr,
                             MemoryRegionSection *section, hwaddr mr_offset)
@@ -1408,8 +1406,9 @@ void *tlb_vaddr_to_host(CPUArchState *env, abi_ptr addr,
  * This almost never fails as the memory access being instrumented
  * should have just filled the TLB. The one corner case is io_writex
  * which can cause TLB flushes and potential resizing of the TLBs
- * loosing the information we need. In those cases we need to recover
- * data from a copy of the io_tlb entry.
+ * losing the information we need. In those cases we need to recover
+ * data from a copy of the iotlbentry. As long as this always occurs
+ * from the same thread (which a mem callback will be) this is safe.
  */
 
 bool tlb_plugin_lookup(CPUState *cpu, target_ulong addr, int mmu_idx,
--
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