The PPC KVM paravirtual interface
The basic execution principle by which KVM on PowerPC works is to run all kernel
space code in PR=1 which is user space. This way we trap all privileged
instructions and can emulate them accordingly.
Unfortunately that is also the downfall. There are quite some privileged
instructions that needlessly return us to the hypervisor even though they
could be handled differently.
This is what the PPC PV interface helps with. It takes privileged instructions
and transforms them into unprivileged ones with some help from the hypervisor.
This cuts down virtualization costs by about 50% on some of my benchmarks.
The code for that interface can be found in arch/powerpc/kernel/kvm*
Querying for existence
To find out if we're running on KVM or not, we leverage the device tree. When
Linux is running on KVM, a node /hypervisor exists. That node contains a
compatible property with the value "linux,kvm".
Once you determined you're running under a PV capable KVM, you can now use
hypercalls as described below.
Inside the device tree's /hypervisor node there's a property called
'hypercall-instructions'. This property contains at most 4 opcodes that make
up the hypercall. To call a hypercall, just call these instructions.
The parameters are as follows:
Register IN OUT
r0 - volatile
r3 1st parameter Return code
r4 2nd parameter 1st output value
r5 3rd parameter 2nd output value
r6 4th parameter 3rd output value
r7 5th parameter 4th output value
r8 6th parameter 5th output value
r9 7th parameter 6th output value
r10 8th parameter 7th output value
r11 hypercall number 8th output value
r12 - volatile
Hypercall definitions are shared in generic code, so the same hypercall numbers
apply for x86 and powerpc alike with the exception that each KVM hypercall
also needs to be ORed with the KVM vendor code which is (42 << 16).
Return codes can be as follows:
12 Hypercall not implemented
The magic page
To enable communication between the hypervisor and guest there is a new shared
page that contains parts of supervisor visible register state. The guest can
map this shared page using the KVM hypercall KVM_HC_PPC_MAP_MAGIC_PAGE.
With this hypercall issued the guest always gets the magic page mapped at the
desired location. The first parameter indicates the effective address when the
MMU is enabled. The second parameter indicates the address in real mode, if
applicable to the target. For now, we always map the page to -4096. This way we
can access it using absolute load and store functions. The following
instruction reads the first field of the magic page:
ld rX, -4096(0)
The interface is designed to be extensible should there be need later to add
additional registers to the magic page. If you add fields to the magic page,
also define a new hypercall feature to indicate that the host can give you more
registers. Only if the host supports the additional features, make use of them.
The magic page layout is described by struct kvm_vcpu_arch_shared
Magic page features
When mapping the magic page using the KVM hypercall KVM_HC_PPC_MAP_MAGIC_PAGE,
a second return value is passed to the guest. This second return value contains
a bitmap of available features inside the magic page.
The following enhancements to the magic page are currently available:
KVM_MAGIC_FEAT_SR Maps SR registers r/w in the magic page
For enhanced features in the magic page, please check for the existence of the
feature before using them!
The MSR contains bits that require hypervisor intervention and bits that do
not require direct hypervisor intervention because they only get interpreted
when entering the guest or don't have any impact on the hypervisor's behavior.
The following bits are safe to be set inside the guest:
If any other bit changes in the MSR, please still use mtmsr(d).
The "ld" and "std" instructions are transormed to "lwz" and "stw" instructions
respectively on 32 bit systems with an added offset of 4 to accommodate for big
The following is a list of mapping the Linux kernel performs when running as
guest. Implementing any of those mappings is optional, as the instruction traps
also act on the shared page. So calling privileged instructions still works as
mfmsr rX ld rX, magic_page->msr
mfsprg rX, 0 ld rX, magic_page->sprg0
mfsprg rX, 1 ld rX, magic_page->sprg1
mfsprg rX, 2 ld rX, magic_page->sprg2
mfsprg rX, 3 ld rX, magic_page->sprg3
mfsrr0 rX ld rX, magic_page->srr0
mfsrr1 rX ld rX, magic_page->srr1
mfdar rX ld rX, magic_page->dar
mfdsisr rX lwz rX, magic_page->dsisr
mtmsr rX std rX, magic_page->msr
mtsprg 0, rX std rX, magic_page->sprg0
mtsprg 1, rX std rX, magic_page->sprg1
mtsprg 2, rX std rX, magic_page->sprg2
mtsprg 3, rX std rX, magic_page->sprg3
mtsrr0 rX std rX, magic_page->srr0
mtsrr1 rX std rX, magic_page->srr1
mtdar rX std rX, magic_page->dar
mtdsisr rX stw rX, magic_page->dsisr
mtmsrd rX, 0 b <special mtmsr section>
mtmsr rX b <special mtmsr section>
mtmsrd rX, 1 b <special mtmsrd section>
mtsrin rX, rY b <special mtsrin section>
wrteei [0|1] b <special wrteei section>
Some instructions require more logic to determine what's going on than a load
or store instruction can deliver. To enable patching of those, we keep some
RAM around where we can live translate instructions to. What happens is the
1) copy emulation code to memory
2) patch that code to fit the emulated instruction
3) patch that code to return to the original pc + 4
4) patch the original instruction to branch to the new code
That way we can inject an arbitrary amount of code as replacement for a single
instruction. This allows us to check for pending interrupts when setting EE=1