diff options
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/devel/atomics.rst | 26 | ||||
| -rw-r--r-- | docs/system/i386/xen.rst | 30 | ||||
| -rw-r--r-- | docs/system/target-mips.rst | 14 |
3 files changed, 44 insertions, 26 deletions
diff --git a/docs/devel/atomics.rst b/docs/devel/atomics.rst index 7957310071..633df65a97 100644 --- a/docs/devel/atomics.rst +++ b/docs/devel/atomics.rst @@ -27,7 +27,8 @@ provides macros that fall in three camps: - weak atomic access and manual memory barriers: ``qatomic_read()``, ``qatomic_set()``, ``smp_rmb()``, ``smp_wmb()``, ``smp_mb()``, - ``smp_mb_acquire()``, ``smp_mb_release()``, ``smp_read_barrier_depends()``; + ``smp_mb_acquire()``, ``smp_mb_release()``, ``smp_read_barrier_depends()``, + ``smp_mb__before_rmw()``, ``smp_mb__after_rmw()``; - sequentially consistent atomic access: everything else. @@ -472,7 +473,7 @@ and memory barriers, and the equivalents in QEMU: sequential consistency. - in QEMU, ``qatomic_read()`` and ``qatomic_set()`` do not participate in - the total ordering enforced by sequentially-consistent operations. + the ordering enforced by read-modify-write operations. This is because QEMU uses the C11 memory model. The following example is correct in Linux but not in QEMU: @@ -488,9 +489,24 @@ and memory barriers, and the equivalents in QEMU: because the read of ``y`` can be moved (by either the processor or the compiler) before the write of ``x``. - Fixing this requires an ``smp_mb()`` memory barrier between the write - of ``x`` and the read of ``y``. In the common case where only one thread - writes ``x``, it is also possible to write it like this: + Fixing this requires a full memory barrier between the write of ``x`` and + the read of ``y``. QEMU provides ``smp_mb__before_rmw()`` and + ``smp_mb__after_rmw()``; they act both as an optimization, + avoiding the memory barrier on processors where it is unnecessary, + and as a clarification of this corner case of the C11 memory model: + + +--------------------------------+ + | QEMU (correct) | + +================================+ + | :: | + | | + | a = qatomic_fetch_add(&x, 2);| + | smp_mb__after_rmw(); | + | b = qatomic_read(&y); | + +--------------------------------+ + + In the common case where only one thread writes ``x``, it is also possible + to write it like this: +--------------------------------+ | QEMU (correct) | diff --git a/docs/system/i386/xen.rst b/docs/system/i386/xen.rst index a00523b492..f06765e88c 100644 --- a/docs/system/i386/xen.rst +++ b/docs/system/i386/xen.rst @@ -9,6 +9,8 @@ KVM has support for hosting Xen guests, intercepting Xen hypercalls and event channel (Xen PV interrupt) delivery. This allows guests which expect to be run under Xen to be hosted in QEMU under Linux/KVM instead. +Using the split irqchip is mandatory for Xen support. + Setup ----- @@ -17,14 +19,14 @@ accelerator, for example for Xen 4.10: .. parsed-literal:: - |qemu_system| --accel kvm,xen-version=0x4000a + |qemu_system| --accel kvm,xen-version=0x4000a,kernel-irqchip=split Additionally, virtual APIC support can be advertised to the guest through the ``xen-vapic`` CPU flag: .. parsed-literal:: - |qemu_system| --accel kvm,xen-version=0x4000a --cpu host,+xen_vapic + |qemu_system| --accel kvm,xen-version=0x4000a,kernel-irqchip=split --cpu host,+xen_vapic When Xen support is enabled, QEMU changes hypervisor identification (CPUID 0x40000000..0x4000000A) to Xen. The KVM identification and features are not @@ -33,11 +35,25 @@ moves to leaves 0x40000100..0x4000010A. The Xen platform device is enabled automatically for a Xen guest. This allows a guest to unplug all emulated devices, in order to use Xen PV block and network -drivers instead. Note that until the Xen PV device back ends are enabled to work -with Xen mode in QEMU, that is unlikely to cause significant joy. Linux guests -can be dissuaded from this by adding 'xen_emul_unplug=never' on their command -line, and it can also be noted that AHCI disk controllers are exempt from being -unplugged, as are passthrough VFIO PCI devices. +drivers instead. Under Xen, the boot disk is typically available both via IDE +emulation, and as a PV block device. Guest bootloaders typically use IDE to load +the guest kernel, which then unplugs the IDE and continues with the Xen PV block +device. + +This configuration can be achieved as follows + +.. parsed-literal:: + + |qemu_system| -M pc --accel kvm,xen-version=0x4000a,kernel-irqchip=split \\ + -drive file=${GUEST_IMAGE},if=none,id=disk,file.locking=off -device xen-disk,drive=disk,vdev=xvda \\ + -drive file=${GUEST_IMAGE},index=2,media=disk,file.locking=off,if=ide + +It is necessary to use the pc machine type, as the q35 machine uses AHCI instead +of legacy IDE, and AHCI disks are not unplugged through the Xen PV unplug +mechanism. + +VirtIO devices can also be used; Linux guests may need to be dissuaded from +umplugging them by adding 'xen_emul_unplug=never' on their command line. Properties ---------- diff --git a/docs/system/target-mips.rst b/docs/system/target-mips.rst index 138441bdec..83239fb9df 100644 --- a/docs/system/target-mips.rst +++ b/docs/system/target-mips.rst @@ -8,8 +8,6 @@ endian options, ``qemu-system-mips``, ``qemu-system-mipsel`` ``qemu-system-mips64`` and ``qemu-system-mips64el``. Five different machine types are emulated: -- A generic ISA PC-like machine \"mips\" - - The MIPS Malta prototype board \"malta\" - An ACER Pica \"pica61\". This machine needs the 64-bit emulator. @@ -19,18 +17,6 @@ machine types are emulated: - A MIPS Magnum R4000 machine \"magnum\". This machine needs the 64-bit emulator. -The generic emulation is supported by Debian 'Etch' and is able to -install Debian into a virtual disk image. The following devices are -emulated: - -- A range of MIPS CPUs, default is the 24Kf - -- PC style serial port - -- PC style IDE disk - -- NE2000 network card - The Malta emulation supports the following devices: - Core board with MIPS 24Kf CPU and Galileo system controller |