Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 28 Aug 2023 11:17:03 +0100
Message-ID: <864jkjfp40.wl-maz@kernel.org>
From:   Marc Zyngier <maz@kernel.org>
To:     Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>
Cc:     linux-kernel@vger.kernel.org, kvmarm@lists.linux.dev,
        linux-arm-kernel@lists.infradead.org, eauger@redhat.com,
        miguel.luis@oracle.com, darren@os.amperecomputing.com,
        scott@os.amperecomputing.com,
        Christoffer Dall <Christoffer.Dall@arm.com>
Subject: Re: [PATCH 2/2] KVM: arm64: timers: Adjust CVAL of a ptimer across guest entry and exits
In-Reply-To: <80bce164-4fb9-0126-0ba0-02581be1a0a5@os.amperecomputing.com>
References: <20230817060314.535987-1-gankulkarni@os.amperecomputing.com>
        <20230817060314.535987-3-gankulkarni@os.amperecomputing.com>
        <87bkf6oyyt.wl-maz@kernel.org>
        <0c5fb304-8c69-80c3-6f1e-487828554244@os.amperecomputing.com>
        <86il97ff17.wl-maz@kernel.org>
        <946b0fc2-3264-d7ab-f5e1-7c9e76db6ebf@os.amperecomputing.com>
        <80bce164-4fb9-0126-0ba0-02581be1a0a5@os.amperecomputing.com>
User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue)
 FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/28.2
 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO)
MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue")
Content-Type: text/plain; charset=US-ASCII
Precedence: bulk

On Thu, 24 Aug 2023 07:37:42 +0100,
Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com> wrote:
> 
> >> Now, to the actual patch: I think the way you offset CVAL isn't
> >> great. You should never have to change it on entry, and you should
> >> instead read the correct value from memory. Then, save/restore of CVAL
> >> must be amended to always apply the offset. Can you give the hack
> >> below a go on your HW?
> 
> I tried this and seems not working, this is due to timer save/restore
> are not called for some of the kvm_exit and entry paths(lighter
> switches).

Can you point me to such paths? Are you referring to the ECV handling
of the physical timer registers?

> 
> I tried changing this patch like, Removed cval adjust from the
> kvm_entry and still cval is adjusted on kvm_exit and in
> timer_restore_state function, reduced cval by offset.
> 
> Please let me know, if this is not you intended to try?
> If possible, please share the steps or pseudo code.

What I want to get to is that:

- on entry (TGE having been flipped to 0), the guest's CVAL is always
  reload from memory, because that's the absolute reference. We should
  never load anything else on the CPU.

- on exit (TGE having been flipped to 1), the guest's CVAL is stored
  as the one true value to memory, and the CPU's view is offset by the
  offset.

- the high-level save/restore helpers apply the offsets back and forth
  as if CNTPOFF didn't exist (because that's exactly the case if
  TGE=1).

Now, I'm pretty sure I'm still missing something, but the above is
roughly the scheme I'm trying to follow?

Thanks,

	M.

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