Re: [PATCH 3/3] x86/vmsi: tolerate unsupported MSI address/data fields

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.01.2021 12:06, Roger Pau Monne wrote:
> Plain MSI doesn't allow caching the MSI address and data fields while
> the capability is enabled and not masked, hence we need to allow any
> changes to those fields to update the binding of the interrupt. For
> reference, the same doesn't apply to MSI-X that is allowed to cache
> the data and address fields while the entry is unmasked, see section
> 6.8.3.5 of the PCI Local Bus Specification 3.0.
> 
> Allowing such updates means that a guest can write an invalid address
> (ie: all zeros) and then a valid one, so the PIRQs shouldn't be
> unmapped when the interrupt cannot be bound to the guest, since
> further updates to the address or data fields can result in the
> binding succeeding.

IOW the breakage from the other patch was because rubbish was
written first, and suitable data was written later on? I didn't
think core PCI code in Linux would do such, which would make me
suspect a driver having custom MSI handling code ...

> Modify the vPCI MSI arch helpers to track whether the interrupt is
> bound, and make failures in vpci_msi_update not unmap the PIRQ, so
> that further calls can attempt to bind the PIRQ again.
> 
> Note this requires some modifications to the MSI-X handlers, but there
> shouldn't be any functional changes in that area.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Am I understanding correctly that this change is independent of
the initial 2 patches (where I have reservations), and hence
it could go in ahead of them (or all alone)?

Jan