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:   Tue, 4 Jul 2023 14:41:36 +0100
From:   Sudeep Holla <sudeep.holla@arm.com>
To:     Mostafa Saleh <smostafa@google.com>
Cc:     maz@kernel.org, oliver.upton@linux.dev,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
        linux-kernel@vger.kernel.org, tabba@google.com, qperret@google.com,
        will@kernel.org, catalin.marinas@arm.com, yuzenghui@huawei.com,
        suzuki.poulose@arm.com, james.morse@arm.com, bgardon@google.com,
        gshan@redhat.com
Subject: Re: [PATCH v3] KVM: arm64: Use BTI for nvhe
Message-ID: <20230704134136.a5znw4jupt5yp5kg@bogus>
References: <20230530150845.2856828-1-smostafa@google.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <20230530150845.2856828-1-smostafa@google.com>
Precedence: bulk

On Tue, May 30, 2023 at 03:08:45PM +0000, Mostafa Saleh wrote:
> CONFIG_ARM64_BTI_KERNEL compiles the kernel to support ARMv8.5-BTI.
> However, the nvhe code doesn't make use of it as it doesn't map any
> pages with Guarded Page(GP) bit.
> 
> kvm pgtable code is modified to map executable pages with GP bit
> if BTI is enabled for the kernel.
> 
> At hyp init, SCTLR_EL2.BT is set to 1 to match EL1 configuration
> (SCTLR_EL1.BT1) set in bti_enable().
> 
> One difference between kernel and nvhe code, is that the kernel maps
> .text with GP while nvhe maps all the executable pages, this makes
> nvhe code need to deal with special initialization code coming from
> other executable sections (.idmap.text).
> For this we need to add bti instruction at the beginning of
> __kvm_handle_stub_hvc as it can be called by  __host_hvc through
> branch instruction(br) and unlike SYM_FUNC_START, SYM_CODE_START
> doesn’t add bti instruction at the beginning, and it can’t be modified
> to add it as it is used with vector tables.
> Another solution which is more intrusive is to convert
> __kvm_handle_stub_hvc to a function and inject “bti jc” instead of
> “bti c” in SYM_FUNC_START
>

I was chasing a bug in linux-next yesterday with protected nVHE(pKVM) and
cpuidle enabled. The system fails to boot. I just bisected the issue to this
patch and also saw this patch landed in the linus tree yesterday/today.
Not sure if this is something to do with the fact that pKVM skips to
__kvm_handle_stub_hvc in __host_hvc.

Let me know if you want be to try something.

-- 
Regards,
Sudeep