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

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Message-Id: <20220406083624.38739-1-reet@codelabs.ch>
Date:   Wed,  6 Apr 2022 10:36:23 +0200
From:   Reto Buerki <reet@...elabs.ch>
To:     dwmw2@...radead.org
Cc:     x86@...nel.org, kvm@...r.kernel.org,
        iommu@...ts.linux-foundation.org, joro@...tes.org,
        tglx@...utronix.de, 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

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?

The attached patch fixes the issue for us.

Regards,
- reto