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Re: [PATCH v2] xen/pass-through: merge emulated bits correctly

From: Anthony PERARD
Subject: Re: [PATCH v2] xen/pass-through: merge emulated bits correctly
Date: Fri, 17 Jun 2022 13:42:21 +0100

On Sat, Jun 11, 2022 at 12:43:29PM -0400, Chuck Zmudzinski wrote:
> In xen_pt_config_reg_init(), there is an error in the merging of the
> emulated data with the host value. With the current Qemu, instead of
> merging the emulated bits with the host bits as defined by emu_mask,
> the emulated bits are merged with the host bits as defined by the
> inverse of emu_mask. In some cases, depending on the data in the
> registers on the host, the way the registers are setup, and the
> initial values of the emulated bits, the end result will be that
> the register is initialized with the wrong value.
> To correct this error, use the XEN_PT_MERGE_VALUE macro to help ensure
> the merge is done correctly.
> This correction is needed to resolve Qemu project issue #1061, which
> describes the failure of Xen HVM Linux guests to boot in certain
> configurations with passed through PCI devices, that is, when this error
> disables instead of enables the PCI_STATUS_CAP_LIST bit of the
> PCI_STATUS register of a passed through PCI device, which in turn
> disables the MSI-X capability of the device in Linux guests with the end
> result being that the Linux guest never completes the boot process.
> Fixes: 2e87512eccf3
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1061
> Buglink: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=988333
> Signed-off-by: Chuck Zmudzinski <brchuckz@aol.com>

Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>

Thank you, looks like it's been a long quest to figure this one out.

Anthony PERARD

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