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Re: [PATCH v6 2/4] Add NVMM accelerator: x86 CPU support

From: Paolo Bonzini
Subject: Re: [PATCH v6 2/4] Add NVMM accelerator: x86 CPU support
Date: Thu, 1 Apr 2021 14:48:40 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0

On 01/04/21 14:32, Reinoud Zandijk wrote:
On Thu, Apr 01, 2021 at 10:35:40AM +0200, Paolo Bonzini wrote:
On 31/03/21 22:07, Reinoud Zandijk wrote:
+void nvmm_vcpu_kick(CPUState *cpu);

Not defined anywhere.

Hmmm, indeed. I think its a leftover of the former patch. Good catch.

+    struct sigaction sigact;
+    sigset_t set;
+    /* Install the IPI handler. */
+    memset(&sigact, 0, sizeof(sigact));
+    sigact.sa_handler = nvmm_ipi_signal;
+    sigaction(SIG_IPI, &sigact, NULL);
+    /* Allow IPIs on the current thread. */
+    sigprocmask(SIG_BLOCK, NULL, &set);
+    sigdelset(&set, SIG_IPI);
+    pthread_sigmask(SIG_SETMASK, &set, NULL);
+    /*
+     * We use the nvmm_vcpu_stop() mechanism, and don't use signals.
+     * Nothing to do.
+     */

Since nvmm_vcpu_stop is very similar to KVM's immediate_exit mechanism, I
think you still need to have a dummy signal handler to kick the VM out of
the run loop *if it is in the kernel*.  The signal handler however can just
do nothing.

Are you worried the in-kernel thread will somehow get stuck or halt on exit of
Qemu and left as a zombie?

No, you need all three of these:

- the signal to interrupt a thread that is running the VM

- the exit_request field to interrupt a thread that is running QEMU code

- nvmm_vcpu_stop() to interrupt a thread that is running kernel code but has not yet started running the VM.

Also, can you just drop support for NVMM_USER_VERSION == 1?

Now thats a good suggestion. We could add support for it in the pkgsrc
package. When 9.0 gets retired, we could then retire it there without the need
to patch Qemu again.

If it's still in use in the wild I have no problem keeping it. The difference is small; I was just asking.

diff --git a/target/i386/nvmm/meson.build b/target/i386/nvmm/meson.build
new file mode 100644
index 0000000000..c154e78014
--- /dev/null
+++ b/target/i386/nvmm/meson.build
@@ -0,0 +1,4 @@
+i386_softmmu_ss.add(when: 'CONFIG_NVMM', if_true: files(
+  'nvmm-all.c',
+  'nvmm-accel-ops.c',

The nvmm library should be added here.

I am not sure what you mean by that. You provided a patch for the meson.build
file, will that not suffice?

That patch dropped nvmm from meson.build, here is where it should be added (so that it is conditional on CONFIG_NVMM and qemu-system-arm does not link to libnvmm).


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