x86/hvm: avoid p2m lookups for vlapic accesses

Enterprise / Virtualization / Xen - Tim Deegan [xen.org] - 16 May 2013 06:05 UTC

The LAPIC base address is a known GFN, so we can skip looking up the p2m: we know it should be handled as emulated MMIO. That helps performance in older Windows OSes, which make a _lot_ of TPR accesses.

This will change the behaviour of any OS that maps other memory/devices at its LAPIC address; the new behaviour (the LAPIC mapping always wins) is closer to actual hardware behaviour.

###

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 7c3cb15..bcf9609 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -1361,6 +1361,17 @@ int hvm_hap_nested_page_fault(paddr_t gpa,
}
}

+ /* For the benefit of 32-bit WinXP (& older Windows) on AMD CPUs,
+ * a fast path for LAPIC accesses, skipping the p2m lookup. */
+ if ( !nestedhvm_vcpu_in_guestmode(v)
+ && gfn == PFN_DOWN(vlapic_base_address(vcpu_vlapic(v))) )
+ {
+ if ( !handle_mmio() )
+ hvm_inject_hw_exception(TRAP_gp_fault, 0);
+ rc = 1;
+ goto out;
+ }
+
p2m = p2m_get_hostp2m(v->domain);
mfn = get_gfn_type_access(p2m, gfn, &p2mt, &p2ma,
P2M_ALLOC | (access_w ? P2M_UNSHARE : 0), NULL);
@@ -2471,6 +2482,12 @@ static enum hvm_copy_result __hvm_copy(
gfn = addr >> PAGE_SHIFT;
}

+ /* For the benefit of 32-bit WinXP (& older Windows) on AMD CPUs,
+ * a fast path for LAPIC accesses, skipping the p2m lookup. */
+ if ( !nestedhvm_vcpu_in_guestmode(curr)
+ && gfn == PFN_DOWN(vlapic_base_address(vcpu_vlapic(curr))) )
+ return HVMCOPY_bad_gfn_to_mfn;
+
page = get_page_from_gfn(curr->domain, gfn, &p2mt, P2M_UNSHARE);

if ( p2m_is_paging(p2mt) )

5d43891 x86/hvm: avoid p2m lookups for vlapic accesses.
xen/arch/x86/hvm/hvm.c | 17 +++++++++++++++++
1 file changed, 17 insertions(+)

Upstream: xenbits.xen.org

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