Re: [RFC PATCH 13/21] KVM: X86: Handle private MMIO as shared

Alexey Kardashevskiy
2024-09-06T03:32:00Z

On 3/9/24 15:13, Xu Yilun wrote:
> On Mon, Sep 02, 2024 at 12:22:56PM +1000, Alexey Kardashevskiy wrote:
>>
>>
>> On 31/8/24 02:57, Xu Yilun wrote:
>>> On Fri, Aug 23, 2024 at 11:21:27PM +1000, Alexey Kardashevskiy wrote:
>>>> Currently private MMIO nested page faults are not expected so when such
>>>> fault occurs, KVM tries moving the faulted page from private to shared
>>>> which is not going to work as private MMIO is not backed by memfd.
>>>>
>>>> Handle private MMIO as shared: skip page state change and memfd
>>>
>>> This means host keeps the mapping for private MMIO, which is different
>>> from private memory. Not sure if it is expected, and I want to get
>>> some directions here.
>>
>> There is no other translation table on AMD though, the same NPT. The
>
> Sorry for not being clear, when I say "host mapping" I mean host
> userspace mapping (host CR3 mapping). By using guest_memfd, there is no
> host CR3 mapping for private memory. I'm wondering if we could keep host
> CR3 mapping for private MMIO.
> >> security is enforced by the RMP table. A device says "bar#x is
private" so
>> the host + firmware ensure the each corresponding RMP entry is "assigned" +
>> "validated" and has a correct IDE stream ID and ASID, and the VM's kernel
>> maps it with the Cbit set.
>>
>>> From HW perspective, private MMIO is not intended to be accessed by
>>> host, but the consequence may varies. According to TDISP spec 11.2,
>>> my understanding is private device (known as TDI) should reject the
>>> TLP and transition to TDISP ERROR state. But no further error
>>> reporting or logging is mandated. So the impact to the host system
>>> is specific to each device. In my test environment, an AER
>>> NonFatalErr is reported and nothing more, much better than host
>>> accessing private memory.
>>
>> afair I get an non-fatal RMP fault so the device does not even notice.
>>
>>> On SW side, my concern is how to deal with mmu_notifier. In theory, if
>>> we get pfn from hva we should follow the userspace mapping change. But
>>> that makes no sense. Especially for TDX TEE-IO, private MMIO mapping
>>> in SEPT cannot be changed or invalidated as long as TDI is running.
>>
>>> Another concern may be specific for TDX TEE-IO. Allowing both userspace
>>> mapping and SEPT mapping may be safe for private MMIO, but on
>>> KVM_SET_USER_MEMORY_REGION2, KVM cannot actually tell if a userspace
>>> addr is really for private MMIO. I.e. user could provide shared memory
>>> addr to KVM but declare it is for private MMIO. The shared memory then
>>> could be mapped in SEPT and cause problem.
>>
>> I am missing lots of context here. When you are starting a guest with a
>> passed through device, until the TDISP machinery transitions the TDI into
>> RUN, this TDI's MMIO is shared and mapped everywhere. And after
>
> Yes, that's the situation nowadays. I think if we need to eliminate
> host CR3 mapping for private MMIO, a simple way is we don't allow host
> CR3 mapping at the first place, even for shared pass through. It is
> doable cause:
>
> 1. IIUC, host CR3 mapping for assigned MMIO is only used for pfn
> finding, i.e. host doesn't really (or shouldn't?) access them.

Well, the host userspace might also want to access MMIO via mmap'ed
region if it is, say, DPDK.

> 2. The hint from guest_memfd shows KVM doesn't have to rely on host
> CR3 mapping to find pfn.

True.

>> transitioning to RUN you move mappings from EPT to SEPT?
>
> Mostly correct, TDX move mapping from EPT to SEPT after LOCKED and
> right before RUN.
>
>>
>>> So personally I prefer no host mapping for private MMIO.
>>
>> Nah, cannot skip this step on AMD. Thanks,
>
> Not sure if we are on the same page.

With the above explanation, we are.

> I assume from HW perspective, host
> CR3 mapping is not necessary for NPT/RMP build?

Yeah, the hw does not require that afaik. But the existing code
continues working for AMD, and I am guessing it is still true for your
case too, right? Unless the host userspace tries accessing the private
MMIO and some horrible stuff happens? Thanks,

> Thanks,
> Yilun
>
>>
>>
>>>
>>> Thanks,
>>> Yilun
>>>
>>>> page state tracking.
>>>>
>>>> The MMIO KVM memory slot is still marked as shared as the guest can
>>>> access it as private or shared so marking the MMIO slot as private
>>>> is not going to help.
>>>>
>>>> Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
>>>> ---
>>>> arch/x86/kvm/mmu/mmu.c | 6 +++++-
>>>> 1 file changed, 5 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>>>> index 928cf84778b0..e74f5c3d0821 100644
>>>> --- a/arch/x86/kvm/mmu/mmu.c
>>>> +++ b/arch/x86/kvm/mmu/mmu.c
>>>> @@ -4366,7 +4366,11 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
>>>> {
>>>> bool async;
>>>> - if (fault->is_private)
>>>> + if (fault->slot && fault->is_private && !kvm_slot_can_be_private(fault->slot) &&
>>>> + (vcpu->kvm->arch.vm_type == KVM_X86_SNP_VM))
>>>> + pr_warn("%s: private SEV TIO MMIO fault for fault->gfn=%llx\n",
>>>> + __func__, fault->gfn);
>>>> + else if (fault->is_private)
>>>> return kvm_faultin_pfn_private(vcpu, fault);
>>>> async = false;
>>>> --
>>>> 2.45.2
>>>>
>>>>
>>
>> --
>> Alexey
>>

--
Alexey