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Fix two freezes in the socket CharIo backend (rcv_char_available throttle, blocking rcv_char) #249

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Fix two freezes in the socket CharIo backend (rcv_char_available throttle, blocking rcv_char) #249

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26094bf
serial: the RX duty-cycle throttle wedged the guest -- availability i…
Oct 29, 2026
1d71da8
serial: never block the CPU thread on the debug channel -- MSG_DONTWA…
Oct 29, 2026
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26 changes: 18 additions & 8 deletions devices/serial/chario.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -348,12 +348,15 @@ void CharIoSocket::rcv_disable()

bool CharIoSocket::rcv_char_available()
{
if (consecutivechars >= 15) {
consecutivechars++;
if (consecutivechars >= 800)
consecutivechars = 0;
return 0;
}
// The A04-22 wedge, root-caused 2026-10-29: this throttle returned "no character
// available" for ~785 of every 800 polls whenever the peer's reply traffic was dense
// (consecutivechars >= 15), while the guest's NubDriver_WaitForByte spins with MSR[EE]=0
// waiting for RR0_RX_CHARACTER_AVAILABLE. The nub's debug transaction then never sees its
// reply, the machine parks in the wait, and the uncollected ADB packets are the visible
// wedge (the Cuda starvation flood). The socket buffer already prevents overrun -- the
// kernel holds unread bytes and select() reports them when the guest is ready -- so this
// duty cycle protected nothing and wedged the guest. Be honest: availability is the
// select() truth.
return this->rcv_char_available_now();
}

Expand Down Expand Up @@ -488,9 +491,16 @@ int CharIoSocket::rcv_char(uint8_t *c)
this->rcv_char_available_now();

if (this->acceptfd != -1) {
int received = (int)recv(this->acceptfd, c, 1, 0);
// MSG_DONTWAIT: the emulator's CPU thread must NEVER block on the debug channel.
// A blocking recv here froze the whole machine whenever guest code read the ESCC
// data register while the peer-side debugger had sent nothing (the silent-peer
// boots froze at the splash logo). EAGAIN = no byte pending; the ESCC's
// rx-available bit (rcv_char_available) remains the sole truth the guest polls.
int received = (int)recv(this->acceptfd, c, 1, MSG_DONTWAIT);
if (received == -1) {
LOG_F(INFO, "socket accept read err: %s", strerror(errno));
if (errno != EAGAIN && errno != EWOULDBLOCK) {
LOG_F(INFO, "socket accept read err: %s", strerror(errno));
}
}
else if (received == 1) {
/*
Expand Down