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Roger Pau Monne
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The Xen platform PCI device (vendor ID 0x5853) exposed to x86 HVM guests doesn't have the functionality of a traditional PCI device. The exposed MMIO BAR is used by some guests (including Linux) as a safe place to map foreign memory, including the grant table itself. Traditionally BARs from devices have the uncacheable (UC) cache attribute from the MTRR, to ensure correct functionality of such devices. hvmloader mimics this behavior and sets the MTRR attributes of both the low and high PCI MMIO windows (where BARs of PCI devices reside) as UC in MTRR. This however causes performance issues for users of the Xen platform PCI device BAR, as for the purposes of mapping remote memory there's no need to use the UC attribute. On Intel systems this is worked around by using iPAT, that allows the hypervisor to force the effective cache attribute of a p2m entry regardless of the guest PAT value. AMD however doesn't have an equivalent of iPAT, and guest PAT values are always considered. Linux commit: 41925b105e34 xen: replace xen_remap() with memremap() Attempted to mitigate this by forcing mappings of the grant-table to use the write-back (WB) cache attribute. However Linux memremap() takes MTRRs into account to calculate which PAT type to use, and seeing the MTRR cache attribute for the region being UC the PAT also ends up as UC, regardless of the caller having requested WB. As a workaround to allow current Linux to map the grant-table as WB using memremap() introduce an xl.cfg option (xen_platform_pci_bar_uc=0) that can be used to select whether the Xen platform PCI device BAR will have the UC attribute in MTRR. Such workaround in hvmloader should also be paired with a fix for Linux so it attempts to change the MTRR of the Xen platform PCI device BAR to WB by itself. Overall, the long term solution would be to provide the guest with a safe range in the guest physical address space where mappings to foreign pages can be created. Some vif throughput performance figures provided by Anthoine from a 8 vCPUs, 4GB of RAM HVM guest(s) running on AMD hardware: Without this patch: vm -> dom0: 1.1Gb/s vm -> vm: 5.0Gb/s With the patch: vm -> dom0: 4.5Gb/s vm -> vm: 7.0Gb/s Reported-by: Anthoine Bourgeois <anthoine.bourgeois@vates.tech> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com> Reviewed-by: Oleksii Kurochko<oleksii.kurochko@gmail.com> Acked-by: Jan Beulich <jbeulich@suse.com> # hvmloader Reviewed-by: Anthony PERARD <anthony.perard@vates.tech>
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