Re: [PATCH v3 7/9] KVM: arm/arm64: Only clean the dcache on translation fault

Marc Zyngier
2018-08-23T12:43:19Z

On 23/08/18 13:24, Alexander Graf wrote:
> On 08/23/2018 01:16 PM, Marc Zyngier wrote:
>> On 21/08/18 17:54, Alexander Graf wrote:
>>> On 08/21/2018 05:08 PM, Marc Zyngier wrote:
>>>> On 21/08/18 15:08, Alexander Graf wrote:
>>>>> On 08/21/2018 03:57 PM, Marc Zyngier wrote:
>>>>>> On 21/08/18 14:35, Alexander Graf wrote:
>>>>>>> On 10/23/2017 06:11 PM, Marc Zyngier wrote:
>>>>>>>> The only case where we actually need to perform a dcache maintenance
>>>>>>>> is when we map the page for the first time, and subsequent permission
>>>>>>>> faults do not require cache maintenance. Let's make it conditional
>>>>>>>> on not being a permission fault (and thus a translation fault).
>>>>>>>>
>>>>>>>> Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
>>>>>>>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>>>>>>> This patch unfortunately breaks something on Hi1616 SoCs when running
>>>>>>> 32bit guests. With this patch applied (and thus with 4.18) I get random
>>>>>>> illegal instruction warnings from 32bit code inside VMs. I do not know
>>>>>>> at this point whether this affects other CPUs as well.
>>>>>> Can you please give a few more details?
>>>>>>
>>>>>> - what are the CPUs on this Hi1616? At least a /proc/cpuinfo would help
>>>>> These are A72s:
>>>>>
>>>>> processor    : 0
>>>>> BogoMIPS    : 100.00
>>>>> Features    : fp asimd evtstrm aes pmull sha1 sha2 crc32 cpuid
>>>>> CPU implementer    : 0x41
>>>>> CPU architecture: 8
>>>>> CPU variant    : 0x0
>>>>> CPU part    : 0xd08
>>>>> CPU revision    : 2
>>>>>
>>>>>> - an example of the crash? Is it within the decompressor? After? This
>>>>>> things do matter, given the number of crazy things the 32bit kernel does
>>>>> They are always in user space. My current reproducer is this:
>>>>>
>>>>>   $ while rpm -qa > /dev/null; do :; done
>>>>>
>>>>> If I run this in parallel with something that just populates RAM (dd
>>>>> if=/dev/nvme0n1 of=/dev/null bs=10G) I get an illegal instruction fault
>>>>> within seconds:
>>>>>
>>>>> sh-4.4# while rpm -qa > /dev/null; do true; done
>>>>> Illegal instruction (core dumped)
>>>>>
>>>>>
>>>>>> - a host kernel configuration?
>>>>> Host kernel configuration is just the normal openSUSE one:
>>>>>
>>>>> https://kernel.opensuse.org/cgit/kernel-source/plain/config/arm64/default?h=stable
>>>>>
>>>>>>> If anyone is interested in a reproducer, I have something handy. But for
>>>>>>> now I believe we should just revert this patch.
>>>>>> Before we revert anything, I'd like to understand what is happening.
>>>>> Yeah, I didn't realize the commit is already in since 4.16, so I agree.
>>>>> I'll bisect a bit, but it may take a while.
>>>> Do you mind giving this a try?
>>>>
>>>> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
>>>> index 1d90d79706bd..df8f3d5eaa22 100644
>>>> --- a/virt/kvm/arm/mmu.c
>>>> +++ b/virt/kvm/arm/mmu.c
>>>> @@ -1531,7 +1536,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>>> kvm_set_pfn_dirty(pfn);
>>>> }
>>>>
>>>> - if (fault_status != FSC_PERM)
>>>> + if (fault_status != FSC_PERM || write_fault)
>>>> clean_dcache_guest_page(pfn, PMD_SIZE);
>>>>
>>>> if (exec_fault) {
>>>> @@ -1553,7 +1558,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>>> mark_page_dirty(kvm, gfn);
>>>> }
>>>>
>>>> - if (fault_status != FSC_PERM)
>>>> + if (fault_status != FSC_PERM || write_fault)
>>>> clean_dcache_guest_page(pfn, PAGE_SIZE);
>>>>
>>>> if (exec_fault) {
>>>>
>>>>
>>>> The missing logic is that a write from the guest could have triggered
>>>> a CoW, meaning we definitely need to flush it in that case too. It
>>>> fixes a kvm-unit-test regression here.
>>> This patch unfortunately does not fix the issue. I still see illegal
>>> instructions.
>> [+Dave]
>>
>> That's because what you're observing has nothing to do with caching, but
>> with FP/SIMD trapping instead. Thanks to the guest image you've provided,
>> I've been able to extract the following:
>>
>> [ 3.944273] systemd-udevd (266): undefined instruction: pc=(ptrval)
>> [ 3.945396] CPU: 0 PID: 266 Comm: systemd-udevd Not tainted 4.17.14-2-lpae #1 openSUSE Tumbleweed (unreleased)
>> [ 3.947130] Hardware name: Generic DT based system
>> [ 3.947976] PC is at 0xb6b9397a
>> [ 3.948547] LR is at 0xb6e9a1b0
>> [ 3.958291] pc : [<b6b9397a>] lr : [<b6e9a1b0>] psr: 20070030
>> [ 3.959664] sp : bebe36a0 ip : b6f6ad50 fp : 005e8784
>> [ 3.960601] r10: bebe3814 r9 : bebe36c0 r8 : bebe36bc
>> [ 3.961522] r7 : bebe3814 r6 : 00000074 r5 : 00000000 r4 : 00000000
>> [ 3.962661] r3 : 005f2b60 r2 : 00000074 r1 : 00000000 r0 : bebe3814
>> [ 3.963801] Flags: nzCv IRQs on FIQs on Mode USER_32 ISA Thumb Segment user
>> [ 4.000606] Control: 30c5383d Table: 6a8e7400 DAC: fffffffd
>> [ 4.001659] Code: d1f9 f1a0 0001 4770 (eee0) 1b10
>>
>> maz@flakes:~/armv7-build-fail$ (echo .thumb; echo .inst.w 0xeee01b10) > x.S && arm-linux-gnueabihf-gcc -c x.S && arm-linux-gnueabihf-objdump -d x.o
>>
>> x.o: file format elf32-littlearm
>>
>>
>> Disassembly of section .text:
>>
>> 00000000 <.text>:
>> 0: eee0 1b10 vdup.8 q0, r1
>>
>> A VFP instruction. Given that you've reported that things worked in
>> 4.17 and broke in 4.18, I strongly suspect the new lazy FPSIMD code.
>> Upon inspection, the way we setup trapping for 32bit is a tiny bit
>> suspect.
>>
>> Could you please give this patch a go? My Seattle has been running
>> with it for 30 minutes now, and it is still running (instead of
>> failing with seconds).
>>
>> Thanks,
>>
>> M.
>>
>> From f2d1d43e2429f9269ab6c565440e095ab3a9be89 Mon Sep 17 00:00:00 2001
>> From: Marc Zyngier <marc.zyngier@arm.com>
>> Date: Thu, 23 Aug 2018 11:51:43 +0100
>> Subject: [PATCH] arm64: KVM: Only force FPEXC32_EL2.EN if trapping FPSIMD
>>
>> If trapping FPSIMD in the context of an AArch32 guest, it is critical
>> to set FPEXC32_EL2.EN to 1 so that the trapping is taken to EL2 and
>> not EL1.
>>
>> Conversely, it is just as critical *not* to set FPEXC32_EL2.EN to 1
>> if we're not going to trap FPSIMD, as we then corrupt the existing
>> VFP state.
>>
>> Moving the call to __activate_traps_fpsimd32 to the point where we
>> know for sure that we are going to trap ensures that we don't set that
>> bit spuriously.
>>
>> Fixes: e6b673b741ea ("KVM: arm64: Optimise FPSIMD handling to reduce guest/host thrashing")
>> Cc: Dave Martin <dave.martin@arm.com>
>> Reported-by: Alexander Graf <agraf@suse.de>
>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>
> Very good catch. This patch does indeed fix it. I'm still puzzled why
> reverting the dcache patch actually had any positive effect, but oh well
> ... :)

Well, there is still a bug in that dcache patch, which I've fixed
separately (in a better way than what I sent before), so you may have
hit that as well, but that's not the most visible bug.

> Tested-by: Alexander Graf <agraf@suse.de>

Thanks for that.

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