This fixes the qemu-kvm wrapper we add for convenience
silently not using KVM, when the system would support it
by at least leaving an indication in the log that the build ran
slower because it ran without KVM.
KVM should only be considered abailable if /dev/kvm exists and
is read-writable by the user that is trying to launch it.
The previous check for existance only had the consequence that
on some Linux distributions running VMs with Nix's QEMU only worked
if KVM was NOT installed.
fixes#124371
It's better to fail to build if our version of Meson isn't compatible
with QEMU's, so we'll know something is wrong. Otherwise, we'll get
subtle breakages that only manifest at runtime, which I think might be
what happened in 9e403b19a1 ("qemu: 5.1.0 -> 5.2.0") to necessitate
autoPatchelfHook.
I dropped autoPatchelfHook because it got confused with the debug
objects, and I can't reproduce the pixman linker errors that caused it
to be introduced in #106988 ("qemu: 5.1.0 -> 5.2.0").
first patch fixes crash in 9p code that occasionally happens also in nixos tests
second patch fixes io errors from discards in aio io engine with virtio-scsi
Tested building qemu_kvm, qemu_full, and qemu_test on x86_64-linux.
Also tested booting a VM generated with nixos-rebuild build-vm.
I wasn't able to test building pkgsMusl.qemu_kvm, because of many
build failures in dependencies.
5e25995295 ("qemu: 2.6.1 -> 2.7.0") added this, because the QEMU
build failed without it. That's no longer the case, so we can bring
back stack protection.
It fails on darwin due to missing `patchelf` and the missing ELFs:
```
/nix/store/...-auto-patchelf-hook/nix-support/setup-hook: line 220: -l: command not found
```
In libc++ starting with LLVM8 there's `<version>` include in `cstddef`:
The following things also align:
* QEMU has a file called `VERSION` in repo root
* QEMU prepends repo root to include path in build
* macOS has a case-insensetive filesystem
All of this combined means that `VERSION` file is included as a header.
Working around this be renaming `VERSION` -> `QEMU_VERSION` to resolve ambiguity.
The problem really only appears on `aarch64-darwin`, since on `x86_64-darwin`
there are no C++ files to compile. The workaround is harmless enough to apply.
This change produces the following warning:
```
... configure: line 619: sysctl: command not found
```
It's benign and sysctl is only useful on MacOS X Leopard:
* https://github.com/qemu/qemu/blob/v5.2.0/configure#L615-L621
Leopard is 13 years old and is not supported by Nix.
The sysctl check is removed in qemu master branch already.
Plus aarch64-darwin is coming in #105026, so there's no reason to force x86_64.
