Re: [RFC PATCH 0/9] kvm: implement atomic memslot updates

David Hildenbrand
2022-09-28T12:53:05Z

On 28.09.22 13:14, Maxim Levitsky wrote:
> On Wed, 2022-09-28 at 11:11 +0200, Emanuele Giuseppe Esposito wrote:
>>
>> Am 27/09/2022 um 17:58 schrieb Sean Christopherson:
>>> On Tue, Sep 27, 2022, Emanuele Giuseppe Esposito wrote:
>>>> Am 26/09/2022 um 23:28 schrieb Sean Christopherson:
>>>>> On Mon, Sep 26, 2022, David Hildenbrand wrote:
>>>>>> As Sean said "This is an awful lot of a complexity to take on for something
>>>>>> that appears to be solvable in userspace."
>>>>>
>>>>> And if the userspace solution is unpalatable for whatever reason, I'd like to
>>>>> understand exactly what KVM behavior is problematic for userspace. E.g. the
>>>>> above RHBZ bug should no longer be an issue as the buggy commit has since been
>>>>> reverted.
>>>>
>>>> It still is because I can reproduce the bug, as also pointed out in
>>>> multiple comments below.
>>>
>>> You can reproduce _a_ bug, but it's obviously not the original bug, because the
>>> last comment says:
>>>
>>> Second, indeed the patch was reverted and somehow accepted without generating
>>> too much noise:
>>>
>>> ...
>>>
>>> The underlying issue of course as we both know is still there.
>>>
>>> You might have luck reproducing it with this bug
>>>
>>> https://bugzilla.redhat.com/show_bug.cgi?id=1855298
>>>
>>> But for me it looks like it is 'working' as well, so you might have
>>> to write a unit test to trigger the issue.
>>>
>>>>> If the issue is KVM doing something nonsensical on a code fetch to MMIO, then I'd
>>>>> much rather fix _that_ bug and improve KVM's user exit ABI to let userspace handle
>>>>> the race _if_ userspace chooses not to pause vCPUs.
>>>>>
>>>>
>>>> Also on the BZ they all seem (Paolo included) to agree that the issue is
>>>> non-atomic memslots update.
>>>
>>> Yes, non-atomic memslot likely results in the guest fetching from a GPA without a
>>> memslot. I'm asking for an explanation of exactly what happens when that occurs,
>>> because it should be possible to adjust KVM and/or QEMU to play nice with the
>>> fetch, e.g. to resume the guest until the new memslot is installed, in which case
>>> an atomic update isn't needed.
>>>
>>> I assume the issue is that KVM exits with KVM_EXIT_INTERNAL_ERROR because the
>>> guest is running at CPL=0, and QEMU kills the guest in response. If that's correct,
>>> then that problem can be solved by exiting to userspace with KVM_EXIT_MMIO instead
>>> of KVM_EXIT_INTERNAL_ERROR so that userspace can do something sane in response to
>>> the MMIO code fetch.
>>>
>>> I'm pretty sure this patch will Just Work for QEMU, because QEMU simply resumes
>>> the vCPU if mmio.len==0. It's a bit of a hack, but I don't think it violates KVM's
>>> ABI in any way, and it can even become "official" behavior since KVM x86 doesn't
>>> otherwise exit with mmio.len==0.
>>>
>>> Compile tested only...
>>
>> So basically you are just making KVM catch the failed
>> kvm_vcpu_read_guest_page() by retuning mmio.len = 0 to QEMU which
>> basically ends up in doing nothing and retry again executing the
>> instruction?
>>
>> I wonder if there are some performance implications in this, but it's
>> definitely simpler than what I did.
>>
>> Tested on the same failing machine used for the BZ, fixes the bug.
>>
>> Do you want me to re-send the patch on your behalf (and add probably a
>> small documentation on Documentation/virt/kvm/api.rst)?
>>
>> Emanuele
>>> ---
>>> From: Sean Christopherson <seanjc@google.com>
>>> Date: Tue, 27 Sep 2022 08:16:03 -0700
>>> Subject: [PATCH] KVM: x86: Exit to userspace with zero-length MMIO "read" on
>>> MMIO fetch
>>>
>>> Exit to userspace with KVM_EXIT_MMIO if emulation fails due to not being
>>> able to fetch instruction bytes, e.g. if the resolved GPA isn't backed by
>>> a memslot. If userspace is manipulating memslots without pausing vCPUs,
>>> e.g. to emulate BIOS relocation, then a vCPU may fetch while there is no
>>> valid memslot installed. Depending on guest context, KVM will either
>>> exit to userspace with KVM_EXIT_INTERNAL_ERROR (L1, CPL=0) or simply
>>> resume the guest (L2 or CPL>0), neither of which is desirable as exiting
>>> with "emulation error" effectively kills the VM, and resuming the guest
>>> doesn't provide userspace an opportunity to react the to fetch.
>>>
>>> Use "mmio.len == 0" to indicate "fetch". This is a bit of a hack, but
>>> there is no other way to communicate "fetch" to userspace without
>>> defining an entirely new exit reason, e.g. "mmio.is_write" is a boolean
>>> and not a flag, and there is no known use case for actually supporting
>>> code fetches from MMIO, i.e. there's no need to allow userspace to fill
>>> in the instruction bytes.
>>>
>>> Signed-off-by: Sean Christopherson <seanjc@google.com>
>>> ---
>>> arch/x86/kvm/emulate.c | 2 ++
>>> arch/x86/kvm/kvm_emulate.h | 1 +
>>> arch/x86/kvm/x86.c | 9 ++++++++-
>>> 3 files changed, 11 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
>>> index f092c54d1a2f..e141238d93b0 100644
>>> --- a/arch/x86/kvm/emulate.c
>>> +++ b/arch/x86/kvm/emulate.c
>>> @@ -5353,6 +5353,8 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int
>>> done:
>>> if (rc == X86EMUL_PROPAGATE_FAULT)
>>> ctxt->have_exception = true;
>>> + if (rc == X86EMUL_IO_NEEDED)
>>> + return EMULATION_IO_FETCH;
>>> return (rc != X86EMUL_CONTINUE) ? EMULATION_FAILED : EMULATION_OK;
>>> }
>>>
>>> diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
>>> index 89246446d6aa..3cb2e321fcd2 100644
>>> --- a/arch/x86/kvm/kvm_emulate.h
>>> +++ b/arch/x86/kvm/kvm_emulate.h
>>> @@ -516,6 +516,7 @@ bool x86_page_table_writing_insn(struct x86_emulate_ctxt *ctxt);
>>> #define EMULATION_OK 0
>>> #define EMULATION_RESTART 1
>>> #define EMULATION_INTERCEPTED 2
>>> +#define EMULATION_IO_FETCH 3
>>> void init_decode_cache(struct x86_emulate_ctxt *ctxt);
>>> int x86_emulate_insn(struct x86_emulate_ctxt *ctxt);
>>> int emulator_task_switch(struct x86_emulate_ctxt *ctxt,
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index aa5ab0c620de..7eb72694c601 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -7129,8 +7129,13 @@ static int kvm_fetch_guest_virt(struct x86_emulate_ctxt *ctxt,
>>> bytes = (unsigned)PAGE_SIZE - offset;
>>> ret = kvm_vcpu_read_guest_page(vcpu, gpa >> PAGE_SHIFT, val,
>>> offset, bytes);
>>> - if (unlikely(ret < 0))
>>> + if (unlikely(ret < 0)) {
>>> + vcpu->run->mmio.phys_addr = gpa;
>>> + vcpu->run->mmio.len = 0;
>>> + vcpu->run->mmio.is_write = 0;
>>> + vcpu->run->exit_reason = KVM_EXIT_MMIO;
>>> return X86EMUL_IO_NEEDED;
>>> + }
>>>
>>> return X86EMUL_CONTINUE;
>>> }
>>> @@ -8665,6 +8670,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>>> r = x86_decode_emulated_instruction(vcpu, emulation_type,
>>> insn, insn_len);
>>> if (r != EMULATION_OK) {
>>> + if (r == EMULATION_IO_FETCH)
>>> + return 0;
>>> if ((emulation_type & EMULTYPE_TRAP_UD) ||
>>> (emulation_type & EMULTYPE_TRAP_UD_FORCED)) {
>>> kvm_queue_exception(vcpu, UD_VECTOR);
>>>
>>> base-commit: 39d9b48cc777bdf6d67d01ed24f1f89b13f5fbb2
>>>
>
> Note that AFAIK, there is another case (and probably more), if TDP is disabled,
> and MMU root is in mmio, we kill the guest.
>
>
> mmu_alloc_shadow_roots -> mmu_check_root
>
>
> I used to have few hacks in KVM to cope with this, but AFAIK,
> I gave up on it, because the issue would show up again and again.

IIRC, s390x can have real problems if we temporarily remove a memslot.
In case the emulation/interpretation code tries accessing guest memory
and fails because there is no memslot describing that portion of guest RAM.

Note that resizing/merging/splitting currently shouldn't happen on
s390x, though. But resizing of memslots might happen in the near future
with virtio-mem in QEMU.

--
Thanks,

David / dhildenb