Re: [libvirt] [PATCH 1/2] qemu: fix recording of vCPU pids for MTTCG

On 10/17/18 10:15 AM, Daniel P. Berrangé wrote: > MTTCG is the new multi-threaded impl of TCG which follows > KVM in having one host OS thread per vCPU. Historically > we have discarded all PIDs reported for TCG guests, but > we must now selectively honour this data. > > We don't have anything in the domain XML that indicates > whether a guest is using TCG or MTTCG. While QEMU does > have an option (-accel tcg,thread=single|multi), it is > not desirable to expose this in libvirt. QEMU will > automatically use MTTCG when the host/guest architecture > pairing is known to be safe. Only developers of QEMU TCG > have a strong reason to override this logic. > > Thus we use two sanity checks to decide if the vCPU > PID information is usable. First we see if the PID > duplicates the main emulator PID, and second we see > if the PID duplicates any other vCPUs. > > Signed-off-by: Daniel P. Berrangé <berrange(a)redhat.com&gt; > --- > src/qemu/qemu_domain.c | 81 ++++++++++++++++++++++++++---------------- > 1 file changed, 51 insertions(+), 30 deletions(-) > > diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c > index f00f1b3fdb..c7a0c03e3f 100644 > --- a/src/qemu/qemu_domain.c > +++ b/src/qemu/qemu_domain.c > @@ -10326,9 +10326,10 @@ qemuDomainRefreshVcpuInfo(virQEMUDriverPtr driver, > qemuDomainVcpuPrivatePtr vcpupriv; > qemuMonitorCPUInfoPtr info = NULL; > size_t maxvcpus = virDomainDefGetVcpusMax(vm->def); > - size_t i; > + size_t i, j; > bool hotplug; > bool fast; > + bool validTIDs = true; > int rc; > int ret = -1; > > @@ -10336,6 +10337,8 @@ qemuDomainRefreshVcpuInfo(virQEMUDriverPtr driver, > fast = virQEMUCapsGet(QEMU_DOMAIN_PRIVATE(vm)->qemuCaps, > QEMU_CAPS_QUERY_CPUS_FAST); > > + VIR_DEBUG("Maxvcpus %zu hotplug %d fast query %d", maxvcpus, hotplug, fast); > + > if (qemuDomainObjEnterMonitorAsync(driver, vm, asyncJob) < 0) > return -1; > > @@ -10348,39 +10351,57 @@ qemuDomainRefreshVcpuInfo(virQEMUDriverPtr driver, > if (rc < 0) > goto cleanup; > > + /* > + * The query-cpus[-fast] commands return information > + * about the vCPUs, including the OS level PID that > + * is executing the vCPU. > + * > + * For KVM there is always a 1-1 mapping between > + * vCPUs and host OS PIDs. > + * > + * For TCG things are a little more complicated. > + * > + * - In some cases the vCPUs will all have the same > + * PID as the main emulator thread. > + * - In some cases the first vCPU will have a distinct > + * PID, but other vCPUs will share the emulator thread > + * > + * For MTTCG, things work the same as KVM, with each > + * vCPU getting its own PID. > + * > + * We use the Host OS PIDs for doing vCPU pinning > + * and reporting. The TCG data reporting will result > + * in bad behaviour such as pinning the wrong PID. > + * We must thus detect and discard bogus PID info > + * from TCG, while still honouring the modern MTTCG > + * impl which we can support. > + */ > + for (i = 0; i < maxvcpus && validTIDs; i++) { > + if (info[i].tid == vm->pid) { > + VIR_DEBUG("vCPU[%zu] PID %llu duplicates process", > + i, (unsigned long long)info[i].tid); > + validTIDs = false; > + } > + If !validTIDs does the next loop matter? IOW: Should the above section add a "continue;" since the loop exit would force the exit?

Beyond that the logic and comments look reasonable. I assume since domain XML doesn't care whether MTTCG or TCG is used and things are handled under the covers by QEMU that means there's no migration or save/restore issues. Of course you have a much deeper understanding of the QEMU code than I do! The one other question I'd have is should validTIDs setting be done just once and saved perhaps in the domain private block? There is more than 1 caller (and *Launch can call twice). It's not like it's going to change, right? So doing the same loop from a hotplug path won't matter nor would subsequent reconnects or attaches. So perhaps the validTIDs should be a tristate that only needs to be checked when the value is UNKNOWN. It's not like the loop is that expensive since it's only numeric comparisons, so it doesn't matter. I suppose I can be easily convinced taking this route would be fine, but figured I'd ask.