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Re: Question on dirty sync before kvm memslot removal

From: Paolo Bonzini
Subject: Re: Question on dirty sync before kvm memslot removal
Date: Wed, 1 Apr 2020 01:12:04 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 31/03/20 18:51, Peter Xu wrote:
> On Tue, Mar 31, 2020 at 05:34:43PM +0200, Paolo Bonzini wrote:
>> On 31/03/20 17:23, Peter Xu wrote:
>>> Yeah, I used a new flag because I thought READONLY was a bit tricky to
>>> be used directly here.  The thing is IIUC if guest writes to a
>>> READONLY slot then KVM should either ignore the write or trigger an
>>> error which I didn't check, however here what we want to do is to let
>>> the write to fallback to the userspace so it's neither dropped (we
>>> still want the written data to land gracefully on RAM), nor triggering
>>> an error (because the slot is actually writable).
>> No, writes fall back to userspace with KVM_MEM_READONLY.
> I read that __kvm_write_guest_page() will return -EFAULT when writting
> to the read-only memslot, and e.g. kvm_write_guest_virt_helper() will
> return with X86EMUL_IO_NEEDED, which will be translated into a
> EMULATION_OK in x86_emulate_insn().  Then in x86_emulate_instruction()
> it seems to get a "1" returned (note that I think it does not set
> either vcpu->arch.pio.count or vcpu->mmio_needed).  Does that mean
> it'll retry the write forever instead of quit into the userspace?  I
> may possibly have misread somewhere, though..

We are definitely relying on KVM_MEM_READONLY to exit to userspace, in
order to emulate flash memory.

> However... I think I might find another race with this:
>           main thread                       vcpu thread
>           -----------                       -----------
>                                             dirty GFN1, cached in PML
>                                             ...
>           remove memslot1 of GFN1
>             set slot READONLY (whatever, or INVALID)
>             sync log (NOTE: no GFN1 yet)
>                                             vmexit, flush PML with RCU
>                                             (will flush to old bitmap) 
> <------- [1]
>             delete memslot1 (old bitmap freed)                         
> <------- [2]
>           add memslot2 of GFN1 (memslot2 could be smaller)
>             add memslot2
> I'm not 100% sure, but I think GFN1's dirty bit will be lost though
> it's correctly applied at [1] but quickly freed at [2].

Yes, we probably need to do a mass vCPU kick when a slot is made
READONLY, before KVM_SET_USER_MEMORY_REGION returns (and after releasing
slots_lock).  It makes sense to guarantee that you can't get any more
dirtying after KVM_SET_USER_MEMORY_REGION returns.


>>> I think the whole kick operation is indeed too heavy for this when
>>> with the run_on_cpu() trick, because the thing we want to know (pml
>>> flushing) is actually per-vcpu and no BQL interaction. Do we have/need
>>> a lightweight way to kick one vcpu in synchronous way?  I was
>>> wondering maybe something like responding a "sync kick" request in the
>>> vcpu thread right after KVM_RUN ends (when we don't have BQL yet).
>>> Would that make sense?
>> Not synchronously, because anything synchronous is very susceptible to
>> deadlocks.
> Yeah it's easy to deadlock (I suffer from it...), but besides above
> case (which I really think it's special) I still think unluckily we
> need a synchronous way.  For example, the VGA code will need the
> latest dirty bit information to decide whether to update the screen
> (or it could stall), or the migration code where we need to calculate
> downtime with the current dirty bit information, etc.

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