Re: [PATCH v4 04/19] kvm: arm64: nvhe: Save the SPE context early

From: Suzuki K Poulose <suzuki.poulose@arm.com>
To: Alexandru Elisei <alexandru.elisei@arm.com>,
	linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org, mathieu.poirier@linaro.org,
	mike.leach@linaro.org, anshuman.khandual@arm.com,
	leo.yan@linaro.org, stable@vger.kernel.org,
	Christoffer Dall <christoffer.dall@arm.com>,
	Marc Zyngier <maz@kernel.org>, Will Deacon <will@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH v4 04/19] kvm: arm64: nvhe: Save the SPE context early
Date: Tue, 2 Mar 2021 10:01:20 +0000	[thread overview]
Message-ID: <abd64d8f-8194-635c-307d-f47ff3dbb498@arm.com> (raw)
In-Reply-To: <efc29f9b-22a9-06c5-9ba0-49cb0d9053b3@arm.com>

Hi Alex

On 3/1/21 4:32 PM, Alexandru Elisei wrote:
> Hello Suzuki,
> 
> On 2/25/21 7:35 PM, Suzuki K Poulose wrote:
>> The nvhe hyp saves the SPE context, flushing any unwritten
> 
> Perhaps that can be reworded to "The nVHE world switch code saves [..]".
> 

Sure

> Also, according to my understanding of "context", that means saving *all* the
> related registers. But KVM saves only *one* register, PMSCR_EL1. I think stating
> that KVM disables sampling and drains the buffer would be more accurate.

I understand that the "context" meaning could be interpreted differently. It could
also mean necessary registers. In this case, as such the guest can't access the SPE,
thus saving the "state" of the SPE only involves saving the PMSCR and restoring
the same. But when we get to to enabling the Guest access, this would mean
saving the other registers too. But yes, your suggestion is clearer, will use
that.

> 
>> data before we switch to the guest. But this operation is
>> performed way too late, because :
>>    - The ownership of the SPE is transferred to EL2. i.e,
> 
> I think the Arm ARM defines only the ownership of the SPE *buffer* (buffer owning
> regime), not the ownership of SPE as a whole.

True. While it means the buffer ownership, all registers except the PMBIDR is
inaccessible to an EL, if the buffer is not accessible (i.e, the ownership is
with a higher EL).

> 
>>      using EL2 translations. (MDCR_EL2_E2PB == 0)
> 
> I think "EL2 translations" is bit too vague, EL2 stage 1 translation would be more
> precise, since we're talking only about the nVHE case.

True.

> 
>>    - The guest Stage1 is loaded.
>>
>> Thus the flush could use the host EL1 virtual address,
>> but use the EL2 translations instead. Fix this by
> 
> It's not *could*, it's *will*. The SPE buffer will use the buffer pointer

Well, if there was nothing to flush, or if the SPE had flushed any data before
we entered the EL2, then there wouldn't be anything left with the flush.

> programmed by the host at EL1, but will attempt to translate it using EL2 stage 1
> translation, where it's (probably) not mapped.
> 
>> and before we change the ownership to EL2.
> 
> Same comment about ownership.
> 
>> The restore path is doing the right thing.
>>
>> Fixes: 014c4c77aad7 ("KVM: arm64: Improve debug register save/restore flow")
>> Cc: stable@vger.kernel.org
>> Cc: Christoffer Dall <christoffer.dall@arm.com>
>> Cc: Marc Zyngier <maz@kernel.org>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Alexandru Elisei <alexandru.elisei@arm.com>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
>> ---
>> New patch.
>> ---
>>   arch/arm64/include/asm/kvm_hyp.h   |  5 +++++
>>   arch/arm64/kvm/hyp/nvhe/debug-sr.c | 12 ++++++++++--
>>   arch/arm64/kvm/hyp/nvhe/switch.c   | 12 +++++++++++-
>>   3 files changed, 26 insertions(+), 3 deletions(-)
>>

>>   
>> diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
>> index f3d0e9eca56c..10eed66136a0 100644
>> --- a/arch/arm64/kvm/hyp/nvhe/switch.c
>> +++ b/arch/arm64/kvm/hyp/nvhe/switch.c
>> @@ -192,6 +192,15 @@ int __kvm_vcpu_run(struct kvm_vcpu *vcpu)
>>   	pmu_switch_needed = __pmu_switch_to_guest(host_ctxt);
>>   
>>   	__sysreg_save_state_nvhe(host_ctxt);
>> +	/*
>> +	 * For nVHE, we must save and disable any SPE
>> +	 * buffers, as the translation regime is going
> 
> I'm not sure what "save and disable any SPE buffers" means. The code definitely
> doesn't save anything related to the SPE buffer. Maybe you're trying to say that

Agreed, this could be clearer. "save" implies the state of the SPE buffer, not the
entire buffer as such. It does save the PMSCR_EL1, which controls where the profiling
is permitted. In turn, we disable the profiling at EL1&0, preventing the any further
generation of data written to the buffer.

> it must drain the buffer contents to memory? Also, SPE has only *one* buffer.
> 

The details on what we do exactly are already in the function where
we take the action. So, we don't need to explain those here. The
comment here is there to give a notice on the dependency on other context
operations, in case someone tries to move this code around.

>> +	 * to be loaded with that of the guest. And we must
>> +	 * save host context for SPE, before we change the
>> +	 * ownership to EL2 (via MDCR_EL2_E2PB == 0)  and before
> 
> Same comments about "save host context for SPE" (from what I understand that
> "context" means, KVM doesn't do that) and ownership (SPE doesn't have an
> ownership, the SPE buffer has an owning translation regime).
> 
>> +	 * we load guest Stage1.
>> +	 */
>> +	__debug_save_host_buffers_nvhe(vcpu);
>>   
>>   	__adjust_pc(vcpu);
>>   
>> @@ -234,11 +243,12 @@ int __kvm_vcpu_run(struct kvm_vcpu *vcpu)
>>   	if (vcpu->arch.flags & KVM_ARM64_FP_ENABLED)
>>   		__fpsimd_save_fpexc32(vcpu);
>>   
>> +	__debug_switch_to_host(vcpu);
>>   	/*
>>   	 * This must come after restoring the host sysregs, since a non-VHE
>>   	 * system may enable SPE here and make use of the TTBRs.
>>   	 */
>> -	__debug_switch_to_host(vcpu);
>> +	__debug_restore_host_buffers_nvhe(vcpu);
>>   
>>   	if (pmu_switch_needed)
>>   		__pmu_switch_to_host(host_ctxt);
> 
> The patch looks correct to me. There are several things that are wrong with the
> way KVM drains the SPE buffer in __debug_switch_to_guest():
> 
> 1. The buffer is drained after the guest's stage 1 is loaded in
> __sysreg_restore_state_nvhe() -> __sysreg_restore_el1_state().
> 
> 2. The buffer is drained after HCR_EL2.VM is set in __activate_traps() ->
> ___activate_traps(), which means that the buffer would have use the guest's stage
> 1 + host's stage 2 for address translation if not 3 below.
> 
> 3. And finally, the buffer is drained after MDCR_EL2.E2PB is set to 0b00 in
> __activate_traps() -> __activate_traps_common() (vcpu->arch.mdcr_el2 is computed
> in kvm_arch_vcpu_ioctl_run() -> kvm_arm_setup_debug() before __kvm_vcpu_run(),
> which means that the buffer will end up using the EL2 stage 1 translation because
> of the ISB after sampling is disabled.
> 

Correct.

> Your fix looks correct to me, we drain the buffer and disable event sampling
> before we start restoring any of the state associated with the guest, and we
> re-enable profiling after we restore all the host's state relevant for profiling.

Thanks for the review.

Cheers
Suzuki