Improve TSC offset matching
PATCH 2/9 Improve TSC offset matching
-------- Original Message --------
Subject: [KVM TSC emulation 2/9] Improve TSC offset matching
Date: Mon, 20 Jun 2011 16:59:30 -0700
From: Zachary Amsden <[email protected]>
To: Avi Kivity <[email protected]>, Marcelo Tosatti <[email protected]>, Glauber Costa <[email protected]>, Frank Arnold <[email protected]>, Joerg Roedel <[email protected]>, Jan Kiszka <[email protected]>, [email protected], [email protected], Zachary Amsden <[email protected]>, Avi Kivity <[email protected]>, Marcelo Tosatti <[email protected]>, Glauber Costa <[email protected]>, Frank Arnold <[email protected]>, Joerg Roedel <[email protected]>, Jan Kiszka <[email protected]>, [email protected]
CC: Zachary Amsden <[email protected]>, Zachary Amsden <[email protected]>
There are a few improvements that can be made to the TSC offset matching code. First, we don't need to call the 128-bit multiply (especially on a constant number), the code works much nicer to do computation in nanosecond units.
Second, the way everything is setup with software TSC rate scaling, we currently have per-cpu rates. Obviously this isn't too desirable to use in practice, but if for some reason we do change the rate of all VCPUs at runtime, then reset the TSCs, we will only want to match offsets for VCPUs running at the same rate.
Finally, for the case where we have an unstable host TSC, but rate scaling is being done in hardware, we should call the platform code to compute the TSC offset, so the math is reorganized to recompute the base instead, then transform the base into an offset using the existing API.
--- arch/x86/include/asm/kvm_host.h | 1 + arch/x86/kvm/x86.c | 37 ++++++++++++++++++++++--------------- 2 files changed, 23 insertions(+), 15 deletions(-)
arch/x86/include/asm/kvm_host.h +1 -0
@@ -468,6 +468,7 @@ struct kvm_arch {u64 last_tsc_nsec;u64 last_tsc_offset;u64 last_tsc_write; +u32 last_tsc_khz;struct kvm_xen_hvm_config xen_hvm_config;
arch/x86/kvm/x86.c +22 -15
@@ -1041,33 +1041,39 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)struct kvm *kvm = vcpu->kvm;u64 offset, ns, elapsed;unsigned long flags; -s64 sdiff; +s64 nsdiff;raw_spin_lock_irqsave (&kvm->arch.tsc_write_lock, flags); offset = kvm_x86_ops->compute_tsc_offset (vcpu, data); ns =get_kernel_ns (); elapsed = ns - kvm->arch.last_tsc_nsec; - sdiff = data - kvm->arch.last_tsc_write; -if (sdiff<0 ) - sdiff = -sdiff; + +/* n.b - signed multiplication and division required */ + nsdiff = data - kvm->arch.last_tsc_write; + nsdiff = (nsdiff *1000 ) / vcpu->arch.virtual_tsc_khz; + nsdiff -= elapsed; +if (nsdiff<0 ) + nsdiff = -nsdiff;/* -* Special case: close write to TSC within 5 seconds of -* another CPU is interpreted as an attempt to synchronize -* The 5 seconds is to accommodate host load / swapping as -* well as any reset of TSC during the boot process. -* -* In that case, for a reliable TSC, we can match TSC offsets, -* or make a best guest using elapsed value. -*/ -if (sdiff<nsec_to_cycles (vcpu,5ULL *NSEC_PER_SEC )&& - elapsed<5ULL *NSEC_PER_SEC ) { +* Special case: TSC write with a small delta (1 second) of virtual +* cycle time against real time is interpreted as an attempt to +* synchronize the CPU. +* +* For a reliable TSC, we can match TSC offsets, and for an unstable +* TSC, we add elapsed time in this computation. We could let the +* compensation code attempt to catch up if we fall behind, but +* it's better to try to match offsets from the beginning. +*/ +if (nsdiff<NSEC_PER_SEC && + vcpu->arch.virtual_tsc_khz == kvm->arch.last_tsc_khz) {if (!check_tsc_unstable ()) { offset = kvm->arch.last_tsc_offset;pr_debug ("kvm: matched tsc offset for %llu\n" , data); }else {u64 delta =nsec_to_cycles (vcpu, elapsed); - offset += delta; + data += delta; + offset = kvm_x86_ops->compute_tsc_offset (vcpu, data);pr_debug ("kvm: adjusted tsc offset by %llu\n" , delta); } ns = kvm->arch.last_tsc_nsec;
@@ -1075,6 +1081,7 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data) kvm->arch.last_tsc_nsec = ns; kvm->arch.last_tsc_write = data; kvm->arch.last_tsc_offset = offset; + kvm->arch.last_tsc_khz = vcpu->arch.virtual_tsc_khz; kvm_x86_ops->write_tsc_offset (vcpu, offset);raw_spin_unlock_irqrestore (&kvm->arch.tsc_write_lock, flags);
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1.7.1