[MODERATED] Re: [PATCH] SPTE masking

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 09/08/18 12:13, speck for Paolo Bonzini wrote:
> On 09/08/2018 12:47, speck for Andrew Cooper wrote:
>>>> and the CPUID maxphysaddr hasn't been
>>>> levelled for heterogeneous migration safety
>>> I don't know about Xen PV, but when using EPT you cannot do that, the
>>> maxphyaddr is not virtualizable (obviously not to guest-maxphyaddr >
>>> host-maxphyaddr, but guest-maxphyaddr < host-maxphyaddr cannot be
>>> emulated either).
>> There is nothing wrong with telling a guest it has maxphysaddr smaller
>> than the real maxphysaddr.  Just like CPUID feature levelling, it says
>> "don't go playing there".
>>
>> No software is permitted to rely on the behaviour of reserved bits.
> That's just not true for page tables.  Bits maxphyaddr:51 are documented
> to generate a page fault with the reserved bits set.  In the future the
> behavior may change (unlikely) but it would be keyed against e.g. a new
> CR4 bit.
>
> In fact, Intel has been stashing new functionality in previously ignored
> bits, of course keying the interpretation on bits from CR4 (e.g.
> protection keys) or VMCS execution controls (e.g. EPT mode-based
> execution control aka XS/XU).
>
> I tried emulating guestphysaddr < hostphysaddr in KVM, but generating
> the reserved bits page fault from EPT violations doesn't work.  If the
> host processor thinks the bits are not reserved and generates e.g. a
> present-but-not-writable fault, no EPT violation happens

Sounds like you've got a KVM bug here.  Either an EPT misconfig or
violation is guaranteed to occur here, because you won't have allowed an
EPT mapping at a guest physical address above guest maxphysaddr, would you?

Even with a strict interpretation 51:maxphysaddr, I don't see any
technical limitations with emulating it correctly.  Making this work in
Xen is on my todo list (once I've figured out exactly how Intel and
AMD's implementation of SMAP differ, so I can fix the software pagewalk
to match the hardware it is running on).

~Andrew