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From: Marc Zyngier <maz@kernel.org>
To: Ricardo Koller <ricarkol@google.com>
Cc: kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu,
	pbonzini@redhat.com, drjones@redhat.com,
	alexandru.elisei@arm.com, eric.auger@redhat.com
Subject: Re: [PATCH v2 4/5] KVM: selftests: Add exception handling support for aarch64
Date: Fri, 07 May 2021 15:31:56 +0100	[thread overview]
Message-ID: <877dkapqcj.wl-maz@kernel.org> (raw)
In-Reply-To: <YJBLFVoRmsehRJ1N@google.com>

On Mon, 03 May 2021 20:12:21 +0100,
Ricardo Koller <ricarkol@google.com> wrote:
> 
> On Mon, May 03, 2021 at 11:32:39AM +0100, Marc Zyngier wrote:
> > On Sat, 01 May 2021 00:24:06 +0100,
> > Ricardo Koller <ricarkol@google.com> wrote:

[...]

> > > +	.if \vector >= 8
> > > +	mrs	x1, sp_el0
> > 
> > I'm still a bit perplexed by this. SP_EL0 is never changed, since you
> > always run in handler mode. Therefore, saving/restoring it is only
> > overhead. If an exception handler wants to introspect it, it is
> > already available in the relevant system register.
> > 
> > Or did you have something else in mind for it?
> > 
> 
> Not really. The reason for saving sp_el0 in there was just for
> consistency, so that handlers for both el0 and el1 exceptions could
> get the sp at regs->sp.

We already have sp_el0 consistency by virtue of having it stored in in
a sysreg.

> Restoring sp_el0 might be too much. So, what do you think of this
> v3: we keep the saving of sp_el0 into regs->sp (to keep things the
> same between el0 and el1) and delete the restoring of sp_el0?

To me, the whole purpose of saving some some context is to allow the
exception handling code to run C code and introspect the interrupted
state. But saving things that are not affected by the context change
seems a bit pointless.

One thing I'd like to see though is to save sp_el1 as it was at the
point of the exception (because that is meaningful to get the
exception context -- think of an unaligned EL1 stack for example),
which means correcting the value that gets saved.

So I would suggest to *only* save sp_el1, to always save it
(irrespective of the exception coming from EL0 or EL1), and to save a
retro-corrected value so that the handler can directly know where the
previous stack pointer was.

Thanks,

	M.

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

  parent reply	other threads:[~2021-05-07 14:32 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-30 23:24 [PATCH v2 0/5] KVM: selftests: arm64 exception handling and debug test Ricardo Koller
2021-04-30 23:24 ` [PATCH v2 1/5] KVM: selftests: Rename vm_handle_exception Ricardo Koller
2021-05-03 11:02   ` Andrew Jones
2021-05-06 12:27   ` Auger Eric
2021-04-30 23:24 ` [PATCH v2 2/5] KVM: selftests: Introduce UCALL_UNHANDLED for unhandled vector reporting Ricardo Koller
2021-05-03 11:09   ` Andrew Jones
2021-05-06 12:27     ` Auger Eric
2021-04-30 23:24 ` [PATCH v2 3/5] KVM: selftests: Move GUEST_ASSERT_EQ to utils header Ricardo Koller
2021-05-03 11:31   ` Andrew Jones
2021-04-30 23:24 ` [PATCH v2 4/5] KVM: selftests: Add exception handling support for aarch64 Ricardo Koller
2021-05-03 10:32   ` Marc Zyngier
2021-05-03 19:12     ` Ricardo Koller
2021-05-06 12:30       ` Auger Eric
2021-05-06 19:14         ` Ricardo Koller
2021-05-07 14:08           ` Auger Eric
2021-05-07 17:54             ` Ricardo Koller
2021-05-12  7:27             ` Ricardo Koller
2021-05-12  8:19               ` Auger Eric
2021-05-12  8:33                 ` Marc Zyngier
2021-05-12  8:52                   ` Auger Eric
2021-05-12 16:06                     ` Ricardo Koller
2021-05-12 12:59         ` Zenghui Yu
2021-05-12 13:43           ` Marc Zyngier
2021-05-12 16:03             ` Ricardo Koller
2021-05-12 16:18               ` Marc Zyngier
2021-05-12 21:39                 ` Ricardo Koller
2021-05-07 14:31       ` Marc Zyngier [this message]
2021-05-07 18:02         ` Ricardo Koller
2021-05-03 12:39   ` Andrew Jones
2021-04-30 23:24 ` [PATCH v2 5/5] KVM: selftests: Add aarch64/debug-exceptions test Ricardo Koller
2021-05-03 12:49   ` Andrew Jones
2021-05-24 12:14 ` [PATCH v2 0/5] KVM: selftests: arm64 exception handling and debug test Paolo Bonzini
2021-05-24 12:59   ` Marc Zyngier

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