Message-ID: <CA+EHjTxvufYVA8LQWRKEX7zA0gWLQUHVO2LvwKc5JXVu-XAEEA@mail.gmail.com>
Date: Wed, 21 May 2025 16:21:46 +0100
From: Fuad Tabba <tabba@...gle.com>
To: Vishal Annapurve <vannapurve@...gle.com>
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Subject: Re: [RFC PATCH v2 04/51] KVM: guest_memfd: Introduce
 KVM_GMEM_CONVERT_SHARED/PRIVATE ioctls

Hi Vishal,

On Wed, 21 May 2025 at 15:42, Vishal Annapurve <vannapurve@...gle.com> wrote:
>
> On Wed, May 21, 2025 at 5:36 AM Fuad Tabba <tabba@...gle.com> wrote:
> > ....
> > > When rebooting, the memslots may not yet be bound to the guest_memfd,
> > > but we want to reset the guest_memfd's to private. If we use
> > > KVM_SET_MEMORY_ATTRIBUTES to convert, we'd be forced to first bind, then
> > > convert. If we had a direct ioctl, we don't have this restriction.
> > >
> > > If we do the conversion via vcpu_run() we would be forced to handle
> > > conversions only with a vcpu_run() and only the guest can initiate a
> > > conversion.
> > >
> > > On a guest boot for TDX, the memory is assumed to be private. If the we
> > > gave it memory set as shared, we'd just have a bunch of
> > > KVM_EXIT_MEMORY_FAULTs that slow down boot. Hence on a guest reboot, we
> > > will want to reset the guest memory to private.
> > >
> > > We could say the firmware should reset memory to private on guest
> > > reboot, but we can't force all guests to update firmware.
> >
> > Here is where I disagree. I do think that this is the CoCo guest's
> > responsibility (and by guest I include its firmware) to fix its own
> > state after a reboot. How would the host even know that a guest is
> > rebooting if it's a CoCo guest?
>
> There are a bunch of complexities here, reboot sequence on x86 can be
> triggered using multiple ways that I don't fully understand, but few
> of them include reading/writing to "reset register" in MMIO/PCI config
> space that are emulated by the host userspace directly. Host has to
> know when the guest is shutting down to manage it's lifecycle.

In that case, I think we need to fully understand these complexities
before adding new IOCTLs. It could be that once we understand these
issues, we find that we don't need these IOCTLs. It's hard to justify
adding an IOCTL for something we don't understand.

> x86 CoCo VM firmwares don't support warm/soft reboot and even if it
> does in future, guest kernel can choose a different reboot mechanism.
> So guest reboot needs to be emulated by always starting from scratch.
> This sequence needs initial guest firmware payload to be installed
> into private ranges of guest_memfd.
>
> >
> > Either the host doesn't (or cannot even) know that the guest is
> > rebooting, in which case I don't see how having an IOCTL would help.
>
> Host does know that the guest is rebooting.

In that case, that (i.e., the host finding out that the guest is
rebooting) could trigger the conversion back to private. No need for
an IOCTL.

> > Or somehow the host does know that, i.e., via a hypercall that
> > indicates that. In which case, we could have it so that for that type
> > of VM, we would reconvert its pages to private on a reboot.
>
> This possibly could be solved by resetting the ranges to private when
> binding with a memslot of certain VM type. But then Google also has a
> usecase to support intrahost migration where a live VM and associated
> guest_memfd files are bound to new KVM VM and memslots.
>
> Otherwise, we need an additional contract between userspace/KVM to
> intercept/handle guest_memfd range reset.

Then this becomes a migration issue to be solved then, not a huge page
support issue. If such IOCTLs are needed for migration, it's too early
to add them now.

Cheers,
/fuad

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