Re: [PATCH v9 3/8] KVM: Add KVM_EXIT_MEMORY_FAULT exit

Chao Peng
2022-11-22T09:55:10Z

On Fri, Nov 18, 2022 at 03:59:12PM +0000, Sean Christopherson wrote:
> On Fri, Nov 18, 2022, Alex Benn?e wrote:
> >
> > Chao Peng <chao.p.peng@linux.intel.com> writes:
> >
> > > On Thu, Nov 17, 2022 at 03:08:17PM +0000, Alex Benn?e wrote:
> > >> >> I think this should be explicit rather than implied by the absence of
> > >> >> another flag. Sean suggested you might want flags for RWX failures so
> > >> >> maybe something like:
> > >> >>
> > >> >> KVM_MEMORY_EXIT_SHARED_FLAG_READ (1 << 0)
> > >> >> KVM_MEMORY_EXIT_SHARED_FLAG_WRITE (1 << 1)
> > >> >> KVM_MEMORY_EXIT_SHARED_FLAG_EXECUTE (1 << 2)
> > >> >> KVM_MEMORY_EXIT_FLAG_PRIVATE (1 << 3)
> > >> >
> > >> > Yes, but I would not add 'SHARED' to RWX, they are not share memory
> > >> > specific, private memory can also set them once introduced.
> > >>
> > >> OK so how about:
> > >>
> > >> KVM_MEMORY_EXIT_FLAG_READ (1 << 0)
> > >> KVM_MEMORY_EXIT_FLAG_WRITE (1 << 1)
> > >> KVM_MEMORY_EXIT_FLAG_EXECUTE (1 << 2)
> > >> KVM_MEMORY_EXIT_FLAG_SHARED (1 << 3)
> > >> KVM_MEMORY_EXIT_FLAG_PRIVATE (1 << 4)
> > >
> > > We don't actually need a new bit, the opposite side of private is
> > > shared, i.e. flags with KVM_MEMORY_EXIT_FLAG_PRIVATE cleared expresses
> > > 'shared'.
> >
> > If that is always true and we never expect a 3rd type of memory that is
> > fine. But given we are leaving room for expansion having an explicit bit
> > allows for that as well as making cases of forgetting to set the flags
> > more obvious.
>
> Hrm, I'm on the fence.
>
> A dedicated flag isn't strictly needed, e.g. even if we end up with 3+ types in
> this category, the baseline could always be "private".

The baseline for the current code is actually "shared".

>
> I do like being explicit, and adding a PRIVATE flag costs KVM practically nothing
> to implement and maintain, but evetually we'll up with flags that are paired with
> an implicit state, e.g. see the many #PF error codes in x86. In other words,
> inevitably KVM will need to define the default/base state of the access, at which
> point the base state for SHARED vs. PRIVATE is "undefined".

Current memory conversion for confidential usage is bi-directional so we
already need both private and shared states and if we use one bit for
both "shared" and "private" then we will have to define the default
state, e.g, currently the default state is "shared" when we define

KVM_MEMORY_EXIT_FLAG_PRIVATE (1 << 0)

>
> The RWX bits are in the same boat, e.g. the READ flag isn't strictly necessary.
> I was thinking more of the KVM_SET_MEMORY_ATTRIBUTES ioctl(), which does need
> the full RWX gamut, when I typed out that response.

For KVM_SET_MEMORY_ATTRIBUTES it's reasonable to add RWX bits and match
that to the permission bits definition in EPT entry.

>
> So I would say if we add an explicit READ flag, then we might as well add an explicit
> PRIVATE flag too. But if we omit PRIVATE, then we should omit READ too.

Since we assume the default state is shared, so we actually only need a
PRIVATE flag, e.g. there is no SHARED flag and will ignore the RWX for now.

Chao