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From: =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= <marcandre.lureau@redhat.com>
Date: Fri, 20 Jul 2018 12:17:59 +0200
Subject: [PATCH 254/268] hw/char/serial: Only retry if qemu_chr_fe_write
 returns 0
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RH-Author: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: <20180720121800.18952-2-marcandre.lureau@redhat.com>
Patchwork-id: 81454
O-Subject: [RHEL-7.6 qemu-kvm-rhev PATCH v2 1/2] hw/char/serial: Only retry if qemu_chr_fe_write returns 0
Bugzilla: 1592817
RH-Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
RH-Acked-by: Markus Armbruster <armbru@redhat.com>
RH-Acked-by: John Snow <jsnow@redhat.com>

From: Sergio Lopez <slp@redhat.com>

Only retry on serial_xmit if qemu_chr_fe_write returns 0, as this is the
only recoverable error.

Retrying with any other scenario, in addition to being a waste of CPU
cycles, can compromise the Guest stability if by the vCPU issuing the
write and the main loop thread are, by chance or explicit pinning,
running on the same pCPU.

Previous discussion:

https://lists.nongnu.org/archive/html/qemu-devel/2018-05/msg06998.html

Signed-off-by: Sergio Lopez <slp@redhat.com>
Message-Id: <1528185295-14199-1-git-send-email-slp@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

(cherry picked from commit 019288bf137183bf3407c9824655b753bfafc99f)
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
---
 hw/char/serial.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/char/serial.c b/hw/char/serial.c
index d6d9b18..d4057bf 100644
--- a/hw/char/serial.c
+++ b/hw/char/serial.c
@@ -262,7 +262,7 @@ static void serial_xmit(SerialState *s)
         if (s->mcr & UART_MCR_LOOP) {
             /* in loopback mode, say that we just received a char */
             serial_receive1(s, &s->tsr, 1);
-        } else if (qemu_chr_fe_write(&s->chr, &s->tsr, 1) != 1 &&
+        } else if (qemu_chr_fe_write(&s->chr, &s->tsr, 1) == 0 &&
                    s->tsr_retry < MAX_XMIT_RETRY) {
             assert(s->watch_tag == 0);
             s->watch_tag =
-- 
1.8.3.1