linux-kernel - Re: [PATCH v3 12/35] x86/msi: Provide msi message shadow structs

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Date:   Thu, 07 Apr 2022 00:07:18 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Reto Buerki <reet@...elabs.ch>, dwmw2@...radead.org
Cc:     x86@...nel.org, kvm@...r.kernel.org,
        iommu@...ts.linux-foundation.org, joro@...tes.org,
        pbonzini@...hat.com, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org, maz@...terjones.org,
        decui@...rosoft.com
Subject: Re: [PATCH v3 12/35] x86/msi: Provide msi message shadow structs

On Wed, Apr 06 2022 at 10:36, Reto Buerki wrote:
> While working on some out-of-tree patches, we noticed that assignment to
> dmar_subhandle of struct x86_msi_data lead to a QEMU warning about
> reserved bits in MSI data being set:
>
> qemu-system-x86_64: vtd_interrupt_remap_msi: invalid IR MSI (sid=256, address=0xfee003d8, data=0x10000)
>
> This message originates from hw/i386/intel_iommu.c in QEMU:
>
> #define VTD_IR_MSI_DATA_RESERVED (0xffff0000)
> if (origin->data & VTD_IR_MSI_DATA_RESERVED) { ... }
>
> Looking at struct x86_msi_data, it appears that it is actually 48-bits in size
> since the bitfield containing the vector, delivery_mode etc is 2 bytes wide
> followed by dmar_subhandle which is 32 bits. Thus assignment to dmar_subhandle
> leads to bits > 16 being set.
>
> If I am not mistaken, the MSI data field should be 32-bits wide for all
> platforms (struct msi_msg, include/linux/msi.h). Is this analysis
> correct or did I miss something wrt. handling of dmar_subhandle?

It's correct and I'm completely surprised that this went unnoticed
for more than a year. Where is that brown paperbag...

Thanks,

        tglx