Re: [PATCH v3 4/4] OvmfPkg: don't assign PCI BARs above 4GiB when CSM enabled

[graf@amazon.com]

Hi David and Laszlo,

(with broken threading because gmane still mirrors the old ML ...)

> Mostly, this is only necessary for devices that the CSM might have
> native support for, such as VirtIO and NVMe; PciBusDxe will already
> degrade devices to 32-bit if they have an OpROM.
> 
> However, there doesn't seem to be a generic way of requesting PciBusDxe
> to downgrade specific devices.
> 
> There's IncompatiblePciDeviceSupportProtocol but that doesn't provide
> the PCI class information or a handle to the device itself, so there's
> no simple way to just match on all NVMe devices, for example.
> 
> Just leave gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio64Size set to zero for
> CSM builds, until/unless that can be fixed.
> 
> Signed-off-by: David Woodhouse <dwmw2@...>
> Reviewed-by: Laszlo Ersek <lersek@...>
> ---
>  OvmfPkg/OvmfPkgIa32X64.dsc | 4 ++++
>  OvmfPkg/OvmfPkgX64.dsc     | 4 ++++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/OvmfPkg/OvmfPkgIa32X64.dsc b/OvmfPkg/OvmfPkgIa32X64.dsc
> index 639e33cb285f..ad20531ceb8b 100644
> --- a/OvmfPkg/OvmfPkgIa32X64.dsc
> +++ b/OvmfPkg/OvmfPkgIa32X64.dsc
> @@ -543,7 +543,11 @@ [PcdsDynamicDefault]
>    gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio32Base|0x0
>    gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio32Size|0x0
>    gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio64Base|0x0
> +!ifdef $(CSM_ENABLE)
> +  gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio64Size|0x0
> +!else
>    gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio64Size|0x800000000
> +!endif
>  
>    gEfiMdePkgTokenSpaceGuid.PcdPlatformBootTimeOut|0
>  
> diff --git a/OvmfPkg/OvmfPkgX64.dsc b/OvmfPkg/OvmfPkgX64.dsc
> index 69a3497c2c9e..0542ac2235b4 100644
> --- a/OvmfPkg/OvmfPkgX64.dsc
> +++ b/OvmfPkg/OvmfPkgX64.dsc
> @@ -542,7 +542,11 @@ [PcdsDynamicDefault]
>    gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio32Base|0x0
>    gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio32Size|0x0
>    gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio64Base|0x0
> +!ifdef $(CSM_ENABLE)
> +  gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio64Size|0x0

IIRC x86 Linux just takes firmware provided BAR maps as they are and 
doesn't map on its own. Or does it map if a BAR was previously unmapped?

In the former case, wouldn't that mean that we're breaking GPU 
passthrough (*big* BARs) for OVMF if the OVMF version happens to support 
CSM? So if a distro decides to turn on CSM, that would be a very subtle 
regression.

Would it be possible to change the PCI mapping logic to just simply 
*prefer* low BAR space if there's some available and the BAR is not big 
(<64MB for example)?

That way we could have CSM enabled OVMF for everyone ;)


Alex


> +!else
>    gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio64Size|0x800000000
> +!endif
>  
>    gEfiMdePkgTokenSpaceGuid.PcdPlatformBootTimeOut|0
>  
> -- 
> 2.21.0