RE: [PATCH v1 0/4] Support dynamic MSI-X allocation

[Liu, Jing2]

Friendly ping to have your valuable inputs and comments. 
Thanks very much.

BRs,
Jing

> On 8/22/2023 3:29 PM, Jing Liu wrote:
> Changes since RFC v1:
> - RFC v1: https://www.mail-archive.com/qemu-
> devel@nongnu.org/msg978637.html
> - Revise the comments. (Alex)
> - Report error of getting irq info and remove the trace of failure
>   case. (Alex, Cédric)
> - Only store dynamic allocation flag as a bool type and test
>   accordingly. (Alex)
> - Move dynamic allocation detection to vfio_msix_early_setup(). (Alex)
> - Change the condition logic in vfio_msix_vector_do_use() that moving
>   the defer_kvm_irq_routing test out and create a common place to update
>   nr_vectors. (Alex)
> - Consolidate the way of MSI-X enabling during device initialization and
>   interrupt restoring that uses fd = -1 trick. Create a function doing
>   that. (Alex)
> 
> Before kernel v6.5, dynamic allocation of MSI-X interrupts was not supported.
> Qemu therefore when allocating a new interrupt, should first release all
> previously allocated interrupts (including disable of MSI-X) and re-allocate 
> all
> interrupts that includes the new one.
> 
> The kernel series [1] adds the support of dynamic MSI-X allocation to vfio-pci
> and uses the existing flag VFIO_IRQ_INFO_NORESIZE to guide user space, that
> when dynamic MSI-X is supported the flag is cleared.
> 
> This series makes the behavior for VFIO PCI devices when dynamic MSI-X
> allocation is supported. When guest unmasks an interrupt, Qemu can directly
> allocate an interrupt on host for this and has nothing to do with the 
> previously
> allocated ones. Therefore, host only allocates interrupts for those unmasked
> (enabled) interrupts inside guest when dynamic MSI-X allocation is supported 
> by
> device.
> 
> When guests enable MSI-X with all of the vectors masked, Qemu need match the
> state to enable MSI-X with no vector enabled. During migration restore, Qemu
> also need enable MSI-X first in dynamic allocation mode, to avoid the guest
> unused vectors being allocated on host. To consolidate them, we use vector 0
> with an invalid fd to get MSI-X enabled and create a common function for this.
> This is cleaner than setting userspace triggering and immediately release.
> 
> Any feedback is appreciated.
> 
> Jing
> 
> [1] https://lwn.net/Articles/931679/
> 
> Jing Liu (4):
>   vfio/pci: detect the support of dynamic MSI-X allocation
>   vfio/pci: enable vector on dynamic MSI-X allocation
>   vfio/pci: use an invalid fd to enable MSI-X
>   vfio/pci: enable MSI-X in interrupt restoring on dynamic allocation
> 
>  hw/vfio/pci.c        | 126 +++++++++++++++++++++++++++++++++----------
>  hw/vfio/pci.h        |   1 +
>  hw/vfio/trace-events |   2 +-
>  3 files changed, 101 insertions(+), 28 deletions(-)
> 
> --
> 2.27.0