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win/vchan: reset the evtchn buffer in case of overflow

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In case of evtchn buffer overflow (received more than 1024 events between
libvchan_wait calls) further reads returns ERROR_IO_DEVICE. The only way to
recover from that is to reset the buffer. Because vchan code doesn't take care
of number of fired events - only the fact that some event was fired - lost
events here shouldn't break anything. Events reported _after_ libvchan_wait
call will be collected and reported correctly.

Some more comments in the code (here and in qrexec-agent in the next commit).
This commit is contained in:
Marek Marczykowski 2012-09-16 23:32:56 +02:00
parent 2444603ef5
commit cb5479666c
1 changed files with 8 additions and 0 deletions
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8
vchan/io.c
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@ -78,6 +78,14 @@ int libvchan_wait(struct libvchan *ctrl)
int ret;
ret = xc_evtchn_pending_with_flush(ctrl->evfd);
// I don't know how to avoid evtchn ring buffer overflow without
// introducing any race condition (in qrexec-agent code). Because of that,
// handle overflow with ring reset - because we just received some events
// (overflow means ring full, so some events was recorded...) the reset
// isn't critical here - always after libvchan_wait we check if there is
// something to read from the vchan
if (ret == -1 && GetLastError() == ERROR_IO_DEVICE)
ret = xc_evtchn_reset(ctrl->evfd);
if (ret!=-1 && xc_evtchn_unmask(ctrl->evfd, ctrl->evport))
return -1;
if (ret!=-1 && libvchan_is_eof(ctrl))