Re: [PATCH 43/59] KVM: arm64: nv: Trap and emulate AT instructions from virtual EL2

Alexandru Elisei
2019-07-18T12:36:45Z

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On 7/18/19 1:13 PM, Tomasz Nowicki wrote:

> Hello Alex,
>
> On 09.07.2019 15:20, Alexandru Elisei wrote:
>> On 6/21/19 10:38 AM, Marc Zyngier wrote:
>>> From: Jintack Lim <jintack.lim@linaro.org>
>>>
>>> When supporting nested virtualization a guest hypervisor executing AT
>>> instructions must be trapped and emulated by the host hypervisor,
>>> because untrapped AT instructions operating on S1E1 will use the wrong
>>> translation regieme (the one used to emulate virtual EL2 in EL1 instead
>> I think that should be "regime".
>>
>>> of virtual EL1) and AT instructions operating on S12 will not work from
>>> EL1.
>>>
>>> This patch does several things.
>>>
>>> 1. List and define all AT system instructions to emulate and document
>>> the emulation design.
>>>
>>> 2. Implement AT instruction handling logic in EL2. This will be used to
>>> emulate AT instructions executed in the virtual EL2.
>>>
>>> AT instruction emulation works by loading the proper processor
>>> context, which depends on the trapped instruction and the virtual
>>> HCR_EL2, to the EL1 virtual memory control registers and executing AT
>>> instructions. Note that ctxt->hw_sys_regs is expected to have the
>>> proper processor context before calling the handling
>>> function(__kvm_at_insn) implemented in this patch.
>>>
>>> 4. Emulate AT S1E[01] instructions by issuing the same instructions in
>>> EL2. We set the physical EL1 registers, NV and NV1 bits as described in
>>> the AT instruction emulation overview.
>> Is item number 3 missing, or is that the result of an unfortunate typo?
>>
>>> 5. Emulate AT A12E[01] instructions in two steps: First, do the stage-1
>>> translation by reusing the existing AT emulation functions. Second, do
>>> the stage-2 translation by walking the guest hypervisor's stage-2 page
>>> table in software. Record the translation result to PAR_EL1.
>>>
>>> 6. Emulate AT S1E2 instructions by issuing the corresponding S1E1
>>> instructions in EL2. We set the physical EL1 registers and the HCR_EL2
>>> register as described in the AT instruction emulation overview.
>>>
>>> 7. Forward system instruction traps to the virtual EL2 if the corresponding
>>> virtual AT bit is set in the virtual HCR_EL2.
>>>
>>> [ Much logic above has been reworked by Marc Zyngier ]
>>>
>>> Signed-off-by: Jintack Lim <jintack.lim@linaro.org>
>>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>>> Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
>>> ---
>>> arch/arm64/include/asm/kvm_arm.h | 2 +
>>> arch/arm64/include/asm/kvm_asm.h | 2 +
>>> arch/arm64/include/asm/sysreg.h | 17 +++
>>> arch/arm64/kvm/hyp/Makefile | 1 +
>>> arch/arm64/kvm/hyp/at.c | 217 +++++++++++++++++++++++++++++++
>>> arch/arm64/kvm/hyp/switch.c | 13 +-
>>> arch/arm64/kvm/sys_regs.c | 202 +++++++++++++++++++++++++++-
>>> 7 files changed, 450 insertions(+), 4 deletions(-)
>>> create mode 100644 arch/arm64/kvm/hyp/at.c
>>>
> [...]
>
>>> +
>>> +void __kvm_at_s1e01(struct kvm_vcpu *vcpu, u32 op, u64 vaddr)
>>> +{
>>> + struct kvm_cpu_context *ctxt = &vcpu->arch.ctxt;
>>> + struct mmu_config config;
>>> + struct kvm_s2_mmu *mmu;
>>> +
>>> + /*
>>> + * We can only get here when trapping from vEL2, so we're
>>> + * translating a guest guest VA.
>>> + *
>>> + * FIXME: Obtaining the S2 MMU for a a guest guest is horribly
>>> + * racy, and we may not find it.
>>> + */
>>> + spin_lock(&vcpu->kvm->mmu_lock);
>>> +
>>> + mmu = lookup_s2_mmu(vcpu->kvm,
>>> + vcpu_read_sys_reg(vcpu, VTTBR_EL2),
>>> + vcpu_read_sys_reg(vcpu, HCR_EL2));
>> From ARM DDI 0487D.b, the description for AT S1E1R (page C5-467, it's the same
>> for the other at s1e{0,1}* instructions):
>>
>> [..] Performs stage 1 address translation, with permisions as if reading from
>> the given virtual address from EL1, or from EL2 [..], using the following
>> translation regime:
>> - If HCR_EL2.{E2H,TGE} is {1, 1}, the EL2&0 translation regime, accessed from EL2.
>>
>> If the guest is VHE, I don't think there's any need to switch mmus. The AT
>> instruction will use the physical EL1&0 translation regime already on the
>> hardware (assuming host HCR_EL2.TGE == 0), which is the vEL2&0 regime for the
>> guest hypervisor.
> Here we want to run AT for L2 (guest guest) EL1&0 regime and not the L1
> (guest hypervisor) so we have to lookup and switch to nested VM MMU
> context. Or did I miss your point?
>
> Thanks,
> Tomasz

What I mean to say is that if the L1 guest has set HCR_EL2.{E2H, TGE} = {1, 1}, then the instruction affects the vEL2&0 translation regime (as per the instruction description in the arhitecture), which is already loaded. The AT instruction will affect the L1 guest hypervisor, not the L2 guest.

In other words:

if (!vcpu_el2_e2h_is_set(vcpu) || !vcpu_el2_tge_is_set(vcpu))
/* switch mmus, the instruction affects the L2 guest (the guest guest) */
else
/* do not switch mmus, the instruction affects the L1 guest hypervisor which is loaded */

I hope this makes things clearer.

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