linux-kernel - Re: [PATCH v2 06/21] arm64: KVM: VHE: Patch out use of HVC

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Message-ID: <56AF5ED8.7010303@arm.com>
Date:	Mon, 1 Feb 2016 13:34:16 +0000
From:	Marc Zyngier <marc.zyngier@....com>
To:	Christoffer Dall <christoffer.dall@...aro.org>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH v2 06/21] arm64: KVM: VHE: Patch out use of HVC

On 01/02/16 13:16, Christoffer Dall wrote:
> On Mon, Jan 25, 2016 at 03:53:40PM +0000, Marc Zyngier wrote:
>> With VHE, the host never issues an HVC instruction to get into the
>> KVM code, as we can simply branch there.
>>
>> Use runtime code patching to simplify things a bit.
>>
>> Signed-off-by: Marc Zyngier <marc.zyngier@....com>
>> ---
>>  arch/arm64/kvm/hyp.S           |  7 +++++++
>>  arch/arm64/kvm/hyp/hyp-entry.S | 38 +++++++++++++++++++++++++++++---------
>>  2 files changed, 36 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
>> index 0ccdcbb..0689a74 100644
>> --- a/arch/arm64/kvm/hyp.S
>> +++ b/arch/arm64/kvm/hyp.S
>> @@ -17,7 +17,9 @@
>>  
>>  #include <linux/linkage.h>
>>  
>> +#include <asm/alternative.h>
>>  #include <asm/assembler.h>
>> +#include <asm/cpufeature.h>
>>  
>>  /*
>>   * u64 kvm_call_hyp(void *hypfn, ...);
>> @@ -38,6 +40,11 @@
>>   * arch/arm64/kernel/hyp_stub.S.
>>   */
>>  ENTRY(kvm_call_hyp)
>> +alternative_if_not ARM64_HAS_VIRT_HOST_EXTN	
>>  	hvc	#0
>>  	ret
>> +alternative_else
>> +	b	__vhe_hyp_call
>> +	nop
>> +alternative_endif
>>  ENDPROC(kvm_call_hyp)
>> diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
>> index 93e8d983..9e0683f 100644
>> --- a/arch/arm64/kvm/hyp/hyp-entry.S
>> +++ b/arch/arm64/kvm/hyp/hyp-entry.S
>> @@ -38,6 +38,32 @@
>>  	ldp	x0, x1, [sp], #16
>>  .endm
>>  
>> +.macro do_el2_call
>> +	/*
>> +	 * Shuffle the parameters before calling the function
>> +	 * pointed to in x0. Assumes parameters in x[1,2,3].
>> +	 */
>> +	stp	lr, xzr, [sp, #-16]!
> 
> remind me why this pair isn't just doing "str" instead of "stp" with the
> xzr ?

Because SP has to be aligned on a 16 bytes boundary at all times.

> 
>> +	mov	lr, x0
>> +	mov	x0, x1
>> +	mov	x1, x2
>> +	mov	x2, x3
>> +	blr	lr
>> +	ldp	lr, xzr, [sp], #16
>> +.endm
>> +
>> +ENTRY(__vhe_hyp_call)
>> +	do_el2_call
>> +	/*
>> +	 * We used to rely on having an exception return to get
>> +	 * an implicit isb. In the E2H case, we don't have it anymore.
>> +	 * rather than changing all the leaf functions, just do it here
>> +	 * before returning to the rest of the kernel.
>> +	 */
> 
> why is this not the case with an ISB before do_el2_call then?

That's a good point. I guess the safest thing to do would be to add one,
but looking at the various functions we call, I don't see any that could
go wrong by not having a ISB in their prologue.

Or maybe you've identified such a case?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...