linux-kernel - Re: [PATCH] ptp: Add vDSO-style vmclock support

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Message-ID: <ebabe603a497391a1671b44ae5579ced281b8333.camel@infradead.org>
Date: Thu, 25 Jul 2024 11:00:15 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Richard Cochran <richardcochran@...il.com>, Peter Hilber
 <peter.hilber@...nsynergy.com>, linux-kernel@...r.kernel.org, 
 virtualization@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org, 
 linux-rtc@...r.kernel.org, "Ridoux, Julien" <ridouxj@...zon.com>, 
 virtio-dev@...ts.linux.dev, "Luu, Ryan" <rluu@...zon.com>, "Chashper,
 David" <chashper@...zon.com>, "Mohamed Abuelfotoh, Hazem"
 <abuehaze@...zon.com>,  "Christopher S . Hall"
 <christopher.s.hall@...el.com>, Jason Wang <jasowang@...hat.com>, John
 Stultz <jstultz@...gle.com>,  netdev@...r.kernel.org, Stephen Boyd
 <sboyd@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Xuan Zhuo
 <xuanzhuo@...ux.alibaba.com>, Marc Zyngier <maz@...nel.org>, Mark Rutland
 <mark.rutland@....com>, Daniel Lezcano <daniel.lezcano@...aro.org>,
 Alessandro Zummo <a.zummo@...ertech.it>,  Alexandre Belloni
 <alexandre.belloni@...tlin.com>, qemu-devel <qemu-devel@...gnu.org>, Simon
 Horman <horms@...nel.org>
Subject: Re: [PATCH] ptp: Add vDSO-style vmclock support

On Thu, 2024-07-25 at 01:54 -0400, Michael S. Tsirkin wrote:
> one other thing worth mentioning is that this design can't work
> with confidential computing setups. By comparison, mapping e.g. a
> range in a PCI BAR would work for these setups.

Why so? This is just like mapping a PCI BAR, isn't it? It's cacheable
MMIO space, *not* part of the encrypted guest RAM ranges. It just
happens to be discovered through the _CRS of an ACPI device, not the
BAR of a PCI device.

> Is there a reason this functionality is not interesting for
> confidential VMs?

It is. In fact, that was one of the reasons for doing it as mappable
MMIO space, instead of having the guest allocate a portion of its own
RAM and invoke a hypervisor enlightenment to populate it. (Although the
latter *can* work with CC too, as demonstrated by e.g. ptp_kvm).

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