[kernel][whp] E: Skip PIC initialization with xAPIC-only mode#13
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esaurez wants to merge 3 commits into
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[kernel][whp] E: Skip PIC initialization with xAPIC-only mode#13esaurez wants to merge 3 commits into
esaurez wants to merge 3 commits into
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- Reduce PIT-based calibration window from 10 ms to 1 ms. - Replace PIT-based calibration with RDTSC spin loop when CPUID leaf 0x16 is available, eliminating ~100 VM exits during boot. - Fall back to PIT-based calibration (1 ms window) when leaf 0x16 is unavailable instead of guessing TSC frequency. - Add max-iteration guard to RDTSC spin loop to prevent hangs. - Fix CPUID helper to check max supported leaf and set ECX=0 explicitly for well-defined subleaf selection.
Hyper-V zeros out CPUID leaf 0x16 on the host, so get_base_frequency_mhz() returns 0. Use WHvGetCapability(ProcessorClockFrequency) instead, which returns the actual TSC frequency in Hz. Without this fix, the guest falls back to PIT-based calibration (184 extra VM exits) instead of the fast RDTSC path. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
When the xAPIC timer has been successfully initialized via try_init_xapic_timer() on WHP+microvm, skip legacy 8259 PIC initialization entirely. The WHP LAPIC emulator handles timer delivery and EOI via MMIO without VM exits. This eliminates ~47 PIC I/O port exits per cold-start (ports 0x20/0x21/0xA0/0xA1), reducing total exits from ~62 to ~15. Each exit costs ~470us on WHP (two hypercalls: DispatchVp + SetVpRegisters), so this saves ~5-22ms depending on system load. The new XapicOnly interrupt controller variant: - Uses xAPIC for EOI (MMIO, handled in-kernel by Hyper-V) - No-ops for unmask (LAPIC timer already unmasked during calibration) - IKC interrupts are injected via WHvRequestInterrupt through the LAPIC, not through PIC routing Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Replica of nanvix#1971.
Summary
Skip legacy 8259 PIC initialization on WHP+microvm when xAPIC is available, reducing VM exits from ~62 to ~15 per cold-start.
Problem
On WHP, every guest I/O port access causes a full VM exit to userspace. PIC initialization writes ICW1-ICW4 commands to ports 0x20/0x21/0xA0/0xA1, totaling ~47 PMIO exits that are handled as no-ops by the VMM.
Implementation
Added XapicOnly(Xapic) variant to InterruptControllerType. Also includes a prerequisite fix: use WHvGetCapability(ProcessorClockFrequency) instead of get_base_frequency_mhz() for the TSC control register.
Measured Improvements (p50)
Depends on #11 (CPUID frequency override).