Linux commit edfe63ec97ed ("x86/mtrr: Fix Xorg crashes in Qemu sessions")
disables PAT feature if MTRR is not enabled. This patch does partial
emulation of MTRR to prevent this from happening: enable fixed-range
MTRRs and disable virable range MTRRs
By default IA32_PAT MSR (SDM Vol3 11.12.4, Table 11-12) doesn't include
'WC' type. If MTRR is disabled from the guests, Linux doesn't allow
writing IA32_PAT MSR so WC type can't be enabled. This creates some
performance issues for certian applications that rely on WC memory type.
Implementation summary:
- Enable MTRR feature: MTRRdefType.E=1
- Enable fixed range MTRRs: MTRRCAP.fix=1, MTRRdefType.FE=1
- For simplicity, disable variable range MTRRs: MTRRCAP.vcnt=0.
It's expected that this bit is honored by the guests and they won't
change the guest memory type through variable MTRRs.
Signed-off-by: bliu11 <baohong.liu@intel.com>
Signed-off-by: Zide Chen <zide.chen@intel.com>
Reviewed-by: Jason Chen CJ <jason.cj.chen@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>
Zide Chen