Subject: [PATCH 01/37] KVM: arm64: Avoid storing the vcpu pointer on the stack

From: [email-masked] (James Morse)
Date: Mon, 27 Nov 2017 11:11:20 +0000

Hi Christoffer,

On 23/11/17 20:59, Christoffer Dall wrote:
> On Thu, Oct 12, 2017 at 04:49:44PM +0100, Marc Zyngier wrote:
>> On 12/10/17 11:41, Christoffer Dall wrote:
>>> We already have the percpu area for the host cpu state, which points to
>>> the VCPU, so there's no need to store the VCPU pointer on the stack on
>>> every context switch.  We can be a little more clever and just use
>>> tpidr_el2 for the percpu offset and load the VCPU pointer from the host
>>> context.
>>>
>>> This requires us to have a scratch register though, so we take the
>>> chance to rearrange some of the el1_sync code to only look at the
>>> vttbr_el2 to determine if this is a trap from the guest or an HVC from
>>> the host.  We do add an extra check to call the panic code if the kernel
>>> is configured with debugging enabled and we saw a trap from the host
>>> which wasn't an HVC, indicating that we left some EL2 trap configured by
>>> mistake.

>>> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
>>> index ab4d0a9..7e48a39 100644
>>> --- a/arch/arm64/include/asm/kvm_asm.h
>>> +++ b/arch/arm64/include/asm/kvm_asm.h
>>> @@ -70,4 +70,24 @@ extern u32 __init_stage2_translation(void);
>>>  
>>>  #endif
>>>  
>>> +#ifdef __ASSEMBLY__
>>> +.macro get_host_ctxt reg, tmp
>>> +	/*
>>> +	 * '=kvm_host_cpu_state' is a host VA from the constant pool, it may
>>> +	 * not be accessible by this address from EL2, hyp_panic() converts
>>> +	 * it with kern_hyp_va() before use.
>>> +	 */
>>
>> This really looks like a stale comment, as there is no hyp_panic
>> involved here anymore (thankfully!).
>>
>>> +	ldr	\reg, =kvm_host_cpu_state
>>> +	mrs	\tmp, tpidr_el2
>>> +	add	\reg, \reg, \tmp

This looks like the arch code's adr_this_cpu.

>>> +	kern_hyp_va \reg
>>
>> Here, we're trading a load from the stack for a load from the constant
>> pool. Can't we do something like:
>>
>> 	adr_l	\reg, kvm_host_cpu_state
>> 	msr	\tmp, tpidr_el2
>> 	add	\reg, \reg, \tmp
>>
>> and that's it? This relies on the property that the kernel/hyp offset is
>> constant, and that it doesn't matter if we add the offset to a kernel VA
>> or a HYP VA... Completely untested of course!
>>
> 
> Coming back to this one, annoyingly, it doesn't seem to work. 

The disassembly looks wrong?, or it generates the wrong address?

> This is the code I use for get_host_ctxt:
> 
> .macro get_host_ctxt reg, tmp
> 	adr_l	\reg, kvm_host_cpu_state
> 	mrs	\tmp, tpidr_el2
> 	add	\reg, \reg, \tmp

(adr_this_cpu)

> 	kern_hyp_va \reg

As we know adr_l used adrp to generate a PC-relative address, when executed at
EL2 it should always generate an EL2 address, so the kern_hyp_va will just mask
out some bits that are already zero.

(this subtly depends on KVM's EL2 code not being a module, and
kvm_host_cpu_state not being percpu_alloc()d)

> .endm
> 
> And this is the disassembly for one of the uses in the hyp code:
> 
> 	adrp	x0, ffff000008ca9000 <overflow_stack+0xd20>
> 	add	x0, x0, #0x7f0
> 	mrs	x1, tpidr_el2
> 	add	x0, x0, x1
> 	and	x0, x0, #0xffffffffffff

(that looks right to me).

> For comparison, the following C-code:
> 
> 	struct kvm_cpu_context *host_ctxt;
> 	host_ctxt = this_cpu_ptr(&kvm_host_cpu_state);
> 	host_ctxt = kern_hyp_va(host_ctxt);
> 
> Gets compiled into this:
> 
> 	adrp	x0, ffff000008ca9000 <overflow_stack+0xd20>
> 	add	x0, x0, #0x7d0
> 	mrs	x1, tpidr_el1
> 	add	x0, x0, #0x20
> 	add	x0, x0, x1
> 	and	x0, x0, #0xffffffffffff

> Any ideas what could be going on here?

You expected tpidr_el2 in the above disassembly?

The patch 'arm64: alternatives: use tpidr_el2 on VHE hosts'[0] wraps the tpidr
access in adr_this_cpu,ldr_this_cpu and __my_cpu_offset() in
ARM64_HAS_VIRT_HOST_EXTN alternatives.

You should have an altinstr_replacement section that contains the 'mrs x1,
tpidr_el2' for this sequence, which will get patched in by the cpufeature code
when we find VHE.

I'm guessing you want to always use tpidr_el2 as cpu_offset for KVM, even on
v8.0 hardware. To do this you can't use the kernel's 'this_cpu_ptr' as its
defined in percpu-defs.h as:
> SHIFT_PERCPU_PTR(ptr, my_cpu_offset)

... and the arch code provides a static-inline 'my_cpu_offset' that resolves to
the correct tpidr for EL1.

I guess you need an asm-accessor for each per-cpu variable you want to access,
or a kvm_this_per_cpu().

> And, during hyp init we do:
> 	mrs	x1, tpidr_el1
> 	msr	tpidr_el2, x1

In the SDEI series this was so that the asm that used tpidr_el2 directly had the
correct value on non-VHE hardware.

Thanks,

James

[0] https://patchwork.kernel.org/patch/10012641/