Re: [PATCH v11 06/10] hvf: arm: Implement -cpu host

[Peter Maydell]

On Thu, 16 Sept 2021 at 17:05, Ard Biesheuvel <ardb@kernel.org> wrote:
> I'd argue that compliance with the architecture means that the
> software should not clear RES1 bits

Architecturally, RES1 means that "software
 * Must not rely on the bit reading as 1.
 * Must use an SBOP policy to write to the bit."
(SBOP=="should be 1 or preserved", ie you can preserve the existing value,
as in "read register, change some bits, write back", or you can write a 1.)

> but I don't think we can blame it
> for not touching bits that were in in invalid state upon entry.

SCTLR_EL1.SPAN == 0 is perfectly valid for a CPU that supports the
PAN feature. It's just not the value OVMF wants, so OVMF should
be setting it to what it does want. Also, as the first thing to
run after reset (ie firmware) OVMF absolutely is responsible for
dealing with system registers which have UNKNOWN values out of
reset.

Put another way, if you want to argue that this is an "invalid
state" you need to point to the specification that defines
the valid state that OVMF should see when it is handed control.

-- PMM