mem_range_attr_set on amd64 returns ENOSPC

[Kevin Zheng]

Hi,

I'm going down a rabbit hole figuring out why i915 drm (12 snapshot but
also earlier) can't set MTRR's on my computer:

Failed to add WC MTRR for [0xe0000000-0xefffffff]: -28; performance may
suffer

(I haven't checked that this address range is sane yet, but even if it
isn't, it should be returning something else.)

That was a return code from a call to mem_range_attr_set. On amd64, that
means x86_mrset is returning -28 (ENOSPC).

That's sys/x86/x86/x86_mem.c returning ENOSPC. But, I'm not familiar
with MTRR's on x86 and why FreeBSD does this search.

There's an ominous comment at the top:

/*
 * Modify/add a variable MTRR to satisfy the request.
 *
 * XXX needs to be updated to properly support "busy" ranges.
 */

Might this have something to do with that?

My system:

FreeBSD 12.0-RELEASE r341666 GENERIC  amd64

CPU: Intel(R) Core(TM) i5-2540M CPU @ 2.60GHz (2591.64-MHz K8-class CPU)
  Origin="GenuineIntel"  Id=0x206a7  Family=0x6  Model=0x2a  Stepping=7

Features=0xbfebfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CLFLUSH,DTS,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT,TM,PBE>

Features2=0x1fbae3ff<SSE3,PCLMULQDQ,DTES64,MON,DS_CPL,VMX,SMX,EST,TM2,SSSE3,CX16,xTPR,PDCM,PCID,SSE4.1,SSE4.2,x2APIC,POPCNT,TSCDLT,AESNI,XSAVE,OSXSAVE,AVX>
  AMD Features=0x28100800<SYSCALL,NX,RDTSCP,LM>
  AMD Features2=0x1<LAHF>
  Structured Extended Features3=0x9c000000<IBPB,STIBP,L1DFL,SSBD>
  XSAVE Features=0x1<XSAVEOPT>
  VT-x: PAT,HLT,MTF,PAUSE,EPT,UG,VPID
  TSC: P-state invariant, performance statistics

Regards,
Kevin

-- 
Kevin Zheng
kevinz5000 at gmail.com | kevinz at berkeley.edu | PGP: 0xC22E1090