Re: [RFC PATCH 09/16] KVM: arm64: Allow ID registers to by dynamically read-as-zero

Christoffer Dall
2018-08-07T19:35:16Z

On Tue, Aug 07, 2018 at 12:09:58PM +0100, Dave Martin wrote:
> On Mon, Aug 06, 2018 at 03:03:24PM +0200, Christoffer Dall wrote:
> > Hi Dave,
> >
> > I think there's a typo in the subject "to be" rather than "to by".
> >
> > On Thu, Jun 21, 2018 at 03:57:33PM +0100, Dave Martin wrote:
> > > When a feature-dependent ID register is hidden from the guest, it
> > > needs to exhibit read-as-zero behaviour as defined by the Arm
> > > architecture, rather than appearing to be entirely absent.
> > >
> > > This patch updates the ID register emulation logic to make use of
> > > the new check_present() method to determine whether the register
> > > should read as zero instead of yielding the host's sanitised
> > > value. Because currently a false result from this method truncates
> > > the trap call chain before the sysreg's emulate method() is called,
> > > a flag is added to distinguish this special case, and helpers are
> > > refactored appropriately.
> >
> > I don't understand this last sentence.
> >
> > And I'm not really sure I understand the code either.
> >
> > I can't seem to see any registers which are defined as !present && !raz,
> > which is what I thought this feature was all about.
>
> !present and !raz is the default behaviour for everything that is not
> ID-register-like. This patch is adding the !present && raz case (though
> that may not be a helpful way to descibe it ... see below).

Fair enough, but I don't really see why you need to classify a register
as !present && raz, because raz implies present AFAICT.

>
> > In other words, what is the benefit of this more generic method as
> > opposed to having a wrapper around read_id_reg() for read_sve_id_reg()
> > which sets RAZ if there is no support for SVE in this context?
>
> There may be other ways to factor this. I can't now remember whay I
> went with this particular approach, except that I vaguely recall
> hitting some obstacles when doing things another way.

What I don't much care for is that we now seem to be mixing the concept
of whether something is present and the value it returns if it is
present in the overall system register handling logic. And I don't
understand why this is a requirement.

>
> Can you take a look at my attempted explanation below and then we
> can reconsider this?

Sure, see my comments below.

>
> [...]
>
> >
> > >
> > > This invloves some trivial updates to pass the vcpu pointer down
> > > into the ID register emulation/access functions.
> > >
> > > A new ID_SANITISED_IF() macro is defined for declaring
> > > conditionally visible ID registers.
> > >
> > > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> > > ---
> > > arch/arm64/kvm/sys_regs.c | 51 ++++++++++++++++++++++++++++++-----------------
> > > arch/arm64/kvm/sys_regs.h | 11 ++++++++++
> > > 2 files changed, 44 insertions(+), 18 deletions(-)
> > >
>
> [...]
>
> > > @@ -1840,7 +1855,7 @@ static int emulate_cp(struct kvm_vcpu *vcpu,
> > >
> > > r = find_reg(params, table, num);
> > >
> > > - if (likely(r) && sys_reg_present(vcpu, r)) {
> > > + if (likely(r) && sys_reg_present_or_raz(vcpu, r)) {
> > > perform_access(vcpu, params, r);
> > > return 0;
> > > }
> > > @@ -2016,7 +2031,7 @@ static int emulate_sys_reg(struct kvm_vcpu *vcpu,
> > > if (!r)
> > > r = find_reg(params, sys_reg_descs, ARRAY_SIZE(sys_reg_descs));
> > >
> > > - if (likely(r) && sys_reg_present(vcpu, r)) {
> > > + if (likely(r) && sys_reg_present_or_raz(vcpu, r)) {
> > > perform_access(vcpu, params, r);
> > > } else {
> > > kvm_err("Unsupported guest sys_reg access at: %lx\n",
> > > @@ -2313,7 +2328,7 @@ int kvm_arm_sys_reg_get_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg
> > > if (!r)
> > > return get_invariant_sys_reg(reg->id, uaddr);
> > >
> > > - if (!sys_reg_present(vcpu, r))
> > > + if (!sys_reg_present_or_raz(vcpu, r))
> > > return -ENOENT;
> > >
> > > if (r->get_user)
> > > @@ -2337,7 +2352,7 @@ int kvm_arm_sys_reg_set_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg
> > > if (!r)
> > > return set_invariant_sys_reg(reg->id, uaddr);
> > >
> > > - if (!sys_reg_present(vcpu, r))
> > > + if (!sys_reg_present_or_raz(vcpu, r))
> > > return -ENOENT;
>
> On Wed, Jul 25, 2018 at 04:46:55PM +0100, Alex Bennée wrote:
> > It's all very well being raz, but shouldn't you catch this further down
> > and not attempt to write the register that doesn't exist?
>
> To be clear, is this a question about factoring, or do you think there's
> a bug here?
>
>
> In response to both sets of comments, I think the way the code is
> factored is causing some confusion.
>
> The idea in my head was something like this:
>
> System register encodings fall into two classes:
>
> a) encodings that we emulate in some way

this is present, then

> b) encodings that we unconditionally reflect back to the guest as an
> Undef.

this is !present, then

The previous change made this a configurable thing as opposed to a
static compile time thing, right?
>
> Architecturally defined system registers fall into two classes:
>
> i) registers whose removal turns all accesses into an Undef
> ii) registers whose removal exhibits some other behaviour.

I'm not sure what you mean by 'removal' here, and which architectural
concept that relates to, which makes it hard for me to parse the rest
here...

>
> These two classifications overlap somwehat.
>
>
> From an emulation perspective, (b), and (i) in the "register not
> present" case, look the same: we trap the register and reflect an Undef
> directly back to the guest with no further action required.
>
> From an emulation perspective, (a) and (ii) are also somewhat the
> same: we need to emulate something, although precisely what we need
> to do depends on which register it is and on whether the register is
> deemed present or not.
>
> sys_reg_check_present_or_raz() thus means "falls under (a) or (ii)",
> i.e., some emulation is required and we need to call sysreg-specific
> methods to figure out precisely what we need to do.

yes, but we've always had that without the "or_raz" stuff at the lookup
level. What has changed?

>
> Conversely !sys_reg_check_present_or_raz() means that we can just
> Undef the guest with no further logic required.

Yes, but that's the same as !present, because raz then implies present,
see above.

>
> Does this rationale make things clearer? The naming is perhaps
> unfortunate.
>

Unfortunately not so much. I have a strong feeling you want to move
anything relating to something being emulated as RAZ/RAO/something else
into sysreg specific functions.

Thanks,
-Christoffer
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