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From: Oliver Upton <oupton@kernel.org>
To: Fuad Tabba <fuad.tabba@linux.dev>
Cc: Marc Zyngier <maz@kernel.org>,
	kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org,
	Joey Gouly <joey.gouly@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Steffen Eiden <seiden@linux.ibm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Quentin Perret <qperret@google.com>,
	Vincent Donnefort <vdonnefort@google.com>,
	Wei-Lin Chang <weilin.chang@arm.com>,
	Fuad Tabba <tabba@google.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/4] KVM: arm64: Don't WARN on an unsupported TLBI OS from vEL1
Date: Mon, 28 Sep 2026 10:05:09 -0700	[thread overview]
Message-ID: <arqeRSIoEwurSrya@kernel.org> (raw)
In-Reply-To: <20260928064643.3265087-2-fuad.tabba@linux.dev>

Hi Fuad,

On Mon, Sep 28, 2026 at 07:46:40AM +0100, Fuad Tabba wrote:
> KVM hides TLBI OS from a guest whose ID registers don't advertise it by
> trapping the instructions: through the fine-grained traps on a CPU with
> FGT, and through HCR_EL2.TTLBOS on one with FEAT_EVT2. With FGT,
> triage_sysreg_trap() makes a trapped TLBI OS UNDEFINED. Without it, the
> instruction reaches handle_tlbi_el1(), which assumes an EL1 TLBI only
> traps from a guest at vEL2 and WARNs before checking whether the guest
> supports it. The guest still gets its UNDEF after the WARN. A VMM can
> trigger the WARN by hiding TLBI OS and having the guest execute one.

So the FGUs don't actually require any hardware support, they're just a
representation of our UNDEF emulation that can be applied even on
non-FGT. That's actually the way we had them wired before the whole
config.c rewrite.

Can you look into reinstating the FGUs?

Thanks,
Oliver

  reply	other threads:[~2026-09-28 17:05 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2026-09-28  6:46 [PATCH v2 0/4] KVM: arm64: Fix HCR_EL2 for non-protected VMs in pKVM Fuad Tabba
2026-09-28  6:46 ` [PATCH v2 1/4] KVM: arm64: Don't WARN on an unsupported TLBI OS from vEL1 Fuad Tabba
2026-09-28 17:05   ` Oliver Upton [this message]
2026-09-28 19:17     ` Fuad Tabba
2026-09-28  6:46 ` [PATCH v2 2/4] KVM: arm64: Clear HCR_EL2.RW for 32-bit non-protected vCPUs Fuad Tabba
2026-09-28  6:46 ` [PATCH v2 3/4] KVM: arm64: Use the host's HCR_EL2 for non-protected VMs in pKVM Fuad Tabba
2026-09-28  6:46 ` [PATCH v2 4/4] KVM: arm64: selftests: Check a feature hidden in an ID register is UNDEF Fuad Tabba
2026-09-28  9:20   ` Venkata Rao Kakani
2026-09-28  9:28     ` Fuad Tabba

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