Re: [RFC PATCH] arm64: unify WORKAROUND_SPECULATIVE_AT_{NVHE,VHE}

From: Marc Zyngier <maz@kernel.org>
To: Will Deacon <will@kernel.org>
Cc: qwandor@google.com, qperret@google.com,
	Suzuki Poulose <Suzuki.Poulose@arm.com>,
	tabba@google.com, Steven Price <steven.price@arm.com>,
	wedsonaf@google.com, Andrew Scull <ascull@google.com>,
	James Morse <James.Morse@arm.com>,
	dbrazdil@google.com, kernel-team@android.com,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC PATCH] arm64: unify WORKAROUND_SPECULATIVE_AT_{NVHE,VHE}
Date: Sat, 28 Mar 2020 11:23:51 +0000	[thread overview]
Message-ID: <20200328112351.53f800ed@why> (raw)
In-Reply-To: <20200327180913.GA10454@willie-the-truck>

On Fri, 27 Mar 2020 18:09:14 +0000
Will Deacon <will@kernel.org> wrote:

> On Fri, Mar 27, 2020 at 05:52:59PM +0000, Marc Zyngier wrote:
> > On 2020-03-27 17:48, Andrew Scull wrote:  
> > > Thanks, Steven. Could we look into the EPD* caching microarch details
> > > Marc was referring to for these A55 and A76 cores?  
> > 
> > Only the folks that have access to the RTL can tell you, unfortunately.
> > 
> > Thankfully, there is a bunch of people on Cc that should be able to ping
> > the relevant teams and report back...  
> 
> Yup, otherwise I guess we can throw in the TLB invalidation after setting
> the EPDx bits until we have clarity from Arm. We wouldn't need to broadcast
> it, so it might not be too bad on performance. If it is, I suppose we could
> switch to a reserved VMID?

I'd like to avoid the reserved VMID if at all possible -- we already
have one for the host on nVHE, and I bet your patches are going to
create some interesting havoc^Wchanges in that area, as the host is now
a guest from the mm perspective. It isn't clear either whether a
reserved VMID really solves the problem either, as you could still
end-up with speculative PTWs. Can it be harmful to create conflicting
TLB entries if you never hit them?

But if we bring in TLB invalidation, I wonder whether the problem can
be mitigated solely with just that on the affected CPUs, and what the
impact would be.

/me eyes the D05 running its 32 guests...

	M.
-- 
Jazz is not dead. It just smells funny...

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