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Date:   Thu, 15 Jun 2023 13:40:57 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Michael Kelley <mikelley@microsoft.com>
Cc:     kys@microsoft.com, haiyangz@microsoft.com, wei.liu@kernel.org,
        decui@microsoft.com, catalin.marinas@arm.com, will@kernel.org,
        tglx@linutronix.de, mingo@redhat.com, dave.hansen@linux.intel.com,
        hpa@zytor.com, linux-kernel@vger.kernel.org,
        linux-hyperv@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        x86@kernel.org
Subject: Re: [PATCH v2 1/2] x86/hyperv: Fix hyperv_pcpu_input_arg handling
 when CPUs go online/offline
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References: <1684862062-51576-1-git-send-email-mikelley@microsoft.com>
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On Tue, May 23, 2023 at 10:14:21AM -0700, Michael Kelley wrote:
> These commits
> 
> a494aef23dfc ("PCI: hv: Replace retarget_msi_interrupt_params with hyperv_pcpu_input_arg")
> 2c6ba4216844 ("PCI: hv: Enable PCI pass-thru devices in Confidential VMs")
> 
> update the Hyper-V virtual PCI driver to use the hyperv_pcpu_input_arg
> because that memory will be correctly marked as decrypted or encrypted
> for all VM types (CoCo or normal). But problems ensue when CPUs in the
> VM go online or offline after virtual PCI devices have been configured.
> 
> When a CPU is brought online, the hyperv_pcpu_input_arg for that CPU is
> initialized by hv_cpu_init() running under state CPUHP_AP_ONLINE_DYN.
> But this state occurs after state CPUHP_AP_IRQ_AFFINITY_ONLINE, which
> may call the virtual PCI driver and fault trying to use the as yet
> uninitialized hyperv_pcpu_input_arg. A similar problem occurs in a CoCo
> VM if the MMIO read and write hypercalls are used from state
> CPUHP_AP_IRQ_AFFINITY_ONLINE.
> 
> When a CPU is taken offline, IRQs may be reassigned in state
> CPUHP_TEARDOWN_CPU. Again, the virtual PCI driver may fault trying to
> use the hyperv_pcpu_input_arg that has already been freed by a
> higher state.
> 
> Fix the onlining problem by adding state CPUHP_AP_HYPERV_ONLINE
> immediately after CPUHP_AP_ONLINE_IDLE (similar to CPUHP_AP_KVM_ONLINE)
> and before CPUHP_AP_IRQ_AFFINITY_ONLINE. Use this new state for
> Hyper-V initialization so that hyperv_pcpu_input_arg is allocated
> early enough.
> 
> Fix the offlining problem by not freeing hyperv_pcpu_input_arg when
> a CPU goes offline. Retain the allocated memory, and reuse it if
> the CPU comes back online later.
> 
> Signed-off-by: Michael Kelley <mikelley@microsoft.com>
> ---
> 
> Changes in v2:
> * Put CPUHP_AP_HYPERV_ONLINE before CPUHP_AP_KVM_ONLINE [Vitaly
>   Kuznetsov]
> 
>  arch/x86/hyperv/hv_init.c  |  2 +-
>  drivers/hv/hv_common.c     | 48 +++++++++++++++++++++++-----------------------
>  include/linux/cpuhotplug.h |  1 +
>  3 files changed, 26 insertions(+), 25 deletions(-)

Acked-by: Borislav Petkov (AMD) <bp@alien8.de>

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette