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Documentation / kvm / ppc-pv.txt

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Based on kernel version Page generated on 2011-06-03 13:47 EST.

1	The PPC KVM paravirtual interface
2	=================================
4	The basic execution principle by which KVM on PowerPC works is to run all kernel
5	space code in PR=1 which is user space. This way we trap all privileged
6	instructions and can emulate them accordingly.
8	Unfortunately that is also the downfall. There are quite some privileged
9	instructions that needlessly return us to the hypervisor even though they
10	could be handled differently.
12	This is what the PPC PV interface helps with. It takes privileged instructions
13	and transforms them into unprivileged ones with some help from the hypervisor.
14	This cuts down virtualization costs by about 50% on some of my benchmarks.
16	The code for that interface can be found in arch/powerpc/kernel/kvm*
18	Querying for existence
19	======================
21	To find out if we're running on KVM or not, we leverage the device tree. When
22	Linux is running on KVM, a node /hypervisor exists. That node contains a
23	compatible property with the value "linux,kvm".
25	Once you determined you're running under a PV capable KVM, you can now use
26	hypercalls as described below.
28	KVM hypercalls
29	==============
31	Inside the device tree's /hypervisor node there's a property called
32	'hypercall-instructions'. This property contains at most 4 opcodes that make
33	up the hypercall. To call a hypercall, just call these instructions.
35	The parameters are as follows:
37		Register	IN			OUT
39		r0		-			volatile
40		r3		1st parameter		Return code
41		r4		2nd parameter		1st output value
42		r5		3rd parameter		2nd output value
43		r6		4th parameter		3rd output value
44		r7		5th parameter		4th output value
45		r8		6th parameter		5th output value
46		r9		7th parameter		6th output value
47		r10		8th parameter		7th output value
48		r11		hypercall number	8th output value
49		r12		-			volatile
51	Hypercall definitions are shared in generic code, so the same hypercall numbers
52	apply for x86 and powerpc alike with the exception that each KVM hypercall
53	also needs to be ORed with the KVM vendor code which is (42 << 16).
55	Return codes can be as follows:
57		Code		Meaning
59		0		Success
60		12		Hypercall not implemented
61		<0		Error
63	The magic page
64	==============
66	To enable communication between the hypervisor and guest there is a new shared
67	page that contains parts of supervisor visible register state. The guest can
68	map this shared page using the KVM hypercall KVM_HC_PPC_MAP_MAGIC_PAGE.
70	With this hypercall issued the guest always gets the magic page mapped at the
71	desired location in effective and physical address space. For now, we always
72	map the page to -4096. This way we can access it using absolute load and store
73	functions. The following instruction reads the first field of the magic page:
75		ld	rX, -4096(0)
77	The interface is designed to be extensible should there be need later to add
78	additional registers to the magic page. If you add fields to the magic page,
79	also define a new hypercall feature to indicate that the host can give you more
80	registers. Only if the host supports the additional features, make use of them.
82	The magic page has the following layout as described in
83	arch/powerpc/include/asm/kvm_para.h:
85	struct kvm_vcpu_arch_shared {
86		__u64 scratch1;
87		__u64 scratch2;
88		__u64 scratch3;
89		__u64 critical;		/* Guest may not get interrupts if == r1 */
90		__u64 sprg0;
91		__u64 sprg1;
92		__u64 sprg2;
93		__u64 sprg3;
94		__u64 srr0;
95		__u64 srr1;
96		__u64 dar;
97		__u64 msr;
98		__u32 dsisr;
99		__u32 int_pending;	/* Tells the guest if we have an interrupt */
100	};
102	Additions to the page must only occur at the end. Struct fields are always 32
103	or 64 bit aligned, depending on them being 32 or 64 bit wide respectively.
105	Magic page features
106	===================
108	When mapping the magic page using the KVM hypercall KVM_HC_PPC_MAP_MAGIC_PAGE,
109	a second return value is passed to the guest. This second return value contains
110	a bitmap of available features inside the magic page.
112	The following enhancements to the magic page are currently available:
114	  KVM_MAGIC_FEAT_SR		Maps SR registers r/w in the magic page
116	For enhanced features in the magic page, please check for the existence of the
117	feature before using them!
119	MSR bits
120	========
122	The MSR contains bits that require hypervisor intervention and bits that do
123	not require direct hypervisor intervention because they only get interpreted
124	when entering the guest or don't have any impact on the hypervisor's behavior.
126	The following bits are safe to be set inside the guest:
128	  MSR_EE
129	  MSR_RI
130	  MSR_CR
131	  MSR_ME
133	If any other bit changes in the MSR, please still use mtmsr(d).
135	Patched instructions
136	====================
138	The "ld" and "std" instructions are transormed to "lwz" and "stw" instructions
139	respectively on 32 bit systems with an added offset of 4 to accommodate for big
140	endianness.
142	The following is a list of mapping the Linux kernel performs when running as
143	guest. Implementing any of those mappings is optional, as the instruction traps
144	also act on the shared page. So calling privileged instructions still works as
145	before.
147	From			To
148	====			==
150	mfmsr	rX		ld	rX, magic_page->msr
151	mfsprg	rX, 0		ld	rX, magic_page->sprg0
152	mfsprg	rX, 1		ld	rX, magic_page->sprg1
153	mfsprg	rX, 2		ld	rX, magic_page->sprg2
154	mfsprg	rX, 3		ld	rX, magic_page->sprg3
155	mfsrr0	rX		ld	rX, magic_page->srr0
156	mfsrr1	rX		ld	rX, magic_page->srr1
157	mfdar	rX		ld	rX, magic_page->dar
158	mfdsisr	rX		lwz	rX, magic_page->dsisr
160	mtmsr	rX		std	rX, magic_page->msr
161	mtsprg	0, rX		std	rX, magic_page->sprg0
162	mtsprg	1, rX		std	rX, magic_page->sprg1
163	mtsprg	2, rX		std	rX, magic_page->sprg2
164	mtsprg	3, rX		std	rX, magic_page->sprg3
165	mtsrr0	rX		std	rX, magic_page->srr0
166	mtsrr1	rX		std	rX, magic_page->srr1
167	mtdar	rX		std	rX, magic_page->dar
168	mtdsisr	rX		stw	rX, magic_page->dsisr
170	tlbsync			nop
172	mtmsrd	rX, 0		b	<special mtmsr section>
173	mtmsr	rX		b	<special mtmsr section>
175	mtmsrd	rX, 1		b	<special mtmsrd section>
177	[Book3S only]
178	mtsrin	rX, rY		b	<special mtsrin section>
180	[BookE only]
181	wrteei	[0|1]		b	<special wrteei section>
184	Some instructions require more logic to determine what's going on than a load
185	or store instruction can deliver. To enable patching of those, we keep some
186	RAM around where we can live translate instructions to. What happens is the
187	following:
189		1) copy emulation code to memory
190		2) patch that code to fit the emulated instruction
191		3) patch that code to return to the original pc + 4
192		4) patch the original instruction to branch to the new code
194	That way we can inject an arbitrary amount of code as replacement for a single
195	instruction. This allows us to check for pending interrupts when setting EE=1
196	for example.
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