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Re: [vfio-users] Supermicro C7Z170-OCE, a rather interesing Skylake Motherboard for VFIO if PEX8747 downstream ACS works





So basically, I would like someone to confirm if PEX8747 ACS support works downstream, even on Intel consumer platforms.

No.  Whether or not a PCIe switch supports ACS is irrelevant when it's placed in a configuration downstream of a PCIe root port that does not support ACS.  The entire path from the endpoint to the IOMMU must support ACS in order for devices to be isolated.


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Sad to hear that, I was really hoping that using the PLX chip could justify its cost by allowing to workaround the lack of ACS isolation on the mainstream consumer platform.



Now that you mention that it doesn't matter if downstream switches support ACS or not, it also means that on Dual GPU cards that uses PLX chips like the GeForce GTX 690, Tesla M60, Radeon 295X2, Radeon 7990 (Which I use as example since its the only one that I could get a good lspci dump from after some googling):


01:00.0 PCI bridge: PLX Technology, Inc. PEX 8747 48-Lane, 5-Port PCI Express Gen 3 (8.0 GT/s) Switch (rev ba)
02:08.0 PCI bridge: PLX Technology, Inc. PEX 8747 48-Lane, 5-Port PCI Express Gen 3 (8.0 GT/s) Switch (rev ba)
02:10.0 PCI bridge: PLX Technology, Inc. PEX 8747 48-Lane, 5-Port PCI Express Gen 3 (8.0 GT/s) Switch (rev ba)
03:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Tahiti XT [Radeon HD 7970/8970 OEM / R9 280X]
03:00.1 Audio device: Advanced Micro Devices, Inc. [AMD/ATI] Tahiti XT HDMI Audio [Radeon HD 7970 Series]
04:00.0 Display controller: Advanced Micro Devices, Inc. [AMD/ATI] Tahiti XT [Radeon HD 7970/8970 OEM / R9 280X]
04:00.1 Audio device: Advanced Micro Devices, Inc. [AMD/ATI] Tahiti XT HDMI Audio [Radeon HD 7970 Series]


...they would ALWAYS be in the same IOMMU Group UNLESS you use them on LGA 2011/2011-3, in which case, the PLX switch ACS support kicks in. Correct?


Also, how does ACS works if coming from the Intel Chipset lanes? I have read that the consumer Chipsets supports a "ACS-like" feature, according to some comments in one of your patches for the ACS quirck. My X10SAT has a PLX chip connected to the Chipset and it seems that I do have proper isolation working there, so switches with ACS connected to the Chipset seems to work downstream.
In a hypothetical case that I drop such Dual GPU card in a slot attached to the Chipset, it means that I would get isolation there, but not if connected to the Processor. Is that correct, too?


BTW, merry christmas.

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