Re: [PATCH v4] PCI: vmd: Disable MSI remap only for low MSI count

[Nirmal Patel <nirmal.patel@linux.intel.com>]

On Thu, 18 Apr 2024 11:31:21 -0400
Nirmal Patel <nirmal.patel@linux.intel.com> wrote:

> VMD MSI remapping is disabled by default for all the CPUs with 28c0
> VMD deviceID. We used to disable remapping because drives supported
> more vectors than the VMD so the performance was better without
> remapping. Now with CPUs that support more than 64 (128 VMD MSIx
> vectors for gen5) we no longer need to disable this feature.
> 
> Note, pci_msix_vec_count() failure is translated to ENODEV per typical
> expectations that drivers may return ENODEV when some driver-known
> fundamental detail of the device is missing.
> 
> Signed-off-by: Nirmal Patel <nirmal.patel@linux.intel.com>
> ---
> v1->v2: Updating commit message.
> v2->v3: Use VMD MSI count instead of cpu count.
> v3->v4: Updating commit message.
> ---
>  drivers/pci/controller/vmd.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/pci/controller/vmd.c
> b/drivers/pci/controller/vmd.c index 769eedeb8802..ba63af70bb63 100644
> --- a/drivers/pci/controller/vmd.c
> +++ b/drivers/pci/controller/vmd.c
> @@ -34,6 +34,8 @@
>  #define MB2_SHADOW_OFFSET	0x2000
>  #define MB2_SHADOW_SIZE		16
>  
> +#define VMD_MIN_MSI_VECTOR_COUNT 64
> +
>  enum vmd_features {
>  	/*
>  	 * Device may contain registers which hint the physical
> location of the @@ -807,13 +809,20 @@ static int
> vmd_enable_domain(struct vmd_dev *vmd, unsigned long features) 
>  	sd->node = pcibus_to_node(vmd->dev->bus);
>  
> +	vmd->msix_count = pci_msix_vec_count(vmd->dev);
> +	if (vmd->msix_count < 0)
> +		return -ENODEV;
> +
>  	/*
>  	 * Currently MSI remapping must be enabled in guest
> passthrough mode
>  	 * due to some missing interrupt remapping plumbing. This is
> probably
>  	 * acceptable because the guest is usually CPU-limited and
> MSI
>  	 * remapping doesn't become a performance bottleneck.
> +	 * Disable MSI remapping only if supported by VMD hardware
> and when
> +	 * VMD MSI count is less than or equal to minimum MSI count.
>  	 */
>  	if (!(features & VMD_FEAT_CAN_BYPASS_MSI_REMAP) ||
> +	    vmd->msix_count > VMD_MIN_MSI_VECTOR_COUNT ||
>  	    offset[0] || offset[1]) {
>  		ret = vmd_alloc_irqs(vmd);
>  		if (ret)

Gentle ping!

Thanks
nirmal