Re: [PATCH] arm64: kvm: handle 52-bit VA regions correctly under nVHE

[Marc Zyngier <maz@kernel.org>]

On Tue, 30 Mar 2021 14:15:19 +0100,
Ard Biesheuvel <ardb@kernel.org> wrote:
> 
> On Tue, 30 Mar 2021 at 15:04, Marc Zyngier <maz@kernel.org> wrote:
> >
> > On Tue, 30 Mar 2021 13:49:18 +0100,
> > Ard Biesheuvel <ardb@kernel.org> wrote:
> > >
> > > On Tue, 30 Mar 2021 at 14:44, Marc Zyngier <maz@kernel.org> wrote:
> > > >
> > > > On Tue, 30 Mar 2021 12:21:26 +0100,
> > > > Ard Biesheuvel <ardb@kernel.org> wrote:
> > > > >
> > > > > Commit f4693c2716b35d08 ("arm64: mm: extend linear region for 52-bit VA
> > > > > configurations") introduced a new layout for the 52-bit VA space, in
> > > > > order to maximize the space available to the linear region. After this
> > > > > change, the kernel VA space is no longer split 1:1 down the middle, and
> > > > > as it turns out, this violates an assumption in the KVM init code when
> > > > > it chooses the layout for the nVHE EL2 mapping.
> > > > >
> > > > > Given that EFI does not support 52-bit VA addressing (as it only
> > > > > supports 4k pages), and that in general, loaders cannot assume that the
> > > > > kernel being loaded supports 52-bit VA/PA addressing in the first place,
> > > > > we can safely assume that the kernel, and therefore the .idmap section,
> > > > > will be 48-bit addressable on 52-bit VA capable systems.
> > > > >
> > > > > So in this case, organize the nVHE EL2 address space as a 2^48 byte
> > > > > window starting at address 0x0, containing the ID map and the
> > > > > hypervisor's private mappings, followed by a contiguous 2^52 - 2^48 byte
> > > > > linear region. (Note that EL1's linear region is 2^52 - 2^47 bytes in
> > > > > size, so it is slightly larger, but this only matters on systems where
> > > > > the DRAM footprint in the physical memory map exceeds 3968 TB)
> > > >
> > > > So if I have memory in the [2^52 - 2^48, 2^52 - 2^47] range, not
> > > > necessarily because I have that much memory, but because my system has
> > > > multiple memory banks, one of which lands on that spot, I cannot map
> > > > such memory at EL2. We'll explode at run time.
> > > >
> > > > Can we keep the private mapping to 47 bits and restore the missing
> > > > chunk to the linear mapping? Of course, it means that the linear map
> > > > is now potential no linear anymore, so we'd have to garantee that the
> > > > kernel lines in the first 2^47 bits instead. Crap.
> > > >
> > >
> > > Yeah. The linear region needs to be contiguous. Alternatively, we
> > > could restrict the upper address limit for loading the kernel to 47
> > > bits.
> >
> > Is that something we can do retroactively? We could mandate it for
> > LVA systems only, but that's a bit odd.
> >
> 
> Yeah, especially given the fact that LVA systems will be VHE capable
> and may therefore not care in the first place.
> 
> On systems that have memory that high, EFI is likely to load the
> kernel there, as it usually allocates from the top down, and it tries
> to avoid having to move it around unless asked to (via KASLR), in
> which case it will currently randomize over the entire available
> memory space.
> 
> So it is going to add a special case for a corner^2 case, i.e., nVHE
> on 52-bit/64k pages with more than 3968 TB distance between the start
> and end of DRAM. Ugh.

Yeah. I'd rather we ignore that memory altogether, but I don't think
we can.

> It seems to me that the only way to solve this is to permit the idmap
> and the hyp linear region to overlap, and use the 2^47 byte window at
> the top of the address space for the hyp private mappings instead of
> the one at the bottom.

But that's the hard problem I want to avoid thinking of.

We need to ensure that there is no EL1 VA that is congruent with the
idmap over the kern_hyp_va() transformation. It means imposing
restrictions over the EL1 linear map, and prevent any allocation that
would result in this overlap (and that is including text).

How do we do that?

Frankly, I think we need to start looking into enabling VHE for the
nVHE /behaviour/. Having a single TTBR on these systems is just
insane.

	M.

-- 
Without deviation from the norm, progress is not possible.

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