drivers: hv: SNP perf fixes

[Brian Perkins (Brian-Perkins)]

Decode register values with vmsa key.

Don't send ipi to cpu that is already pending an interrupt scan.

Check for interrupts handled entirely in the kernel.

Add kernel fast-path for some stimer0 operations.

[Copilot]

Pull request overview

This PR improves Hyper-V VTL (VTL2) performance for SEV-SNP guests by reducing avoidable user/kernel transitions and unnecessary IPIs, and by adding a kernel fast-path for select synthetic timer operations.

Changes:

• Add an SNP per-vCPU context block to the shared mshv_vtl_run page, including a VMSA tweak bitmap and synthetic timer (stimer0) state.
• Coalesce proxy-IRR scan wakeups so CPUs aren’t sent reschedule IPIs when a scan is already pending.
• Add SNP kernel handling for interrupt-entry and for HV_X64_MSR_STIMER0_COUNT writes (including an hrtimer-backed wakeup path).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
drivers/hv/mshv_vtl.h	Introduces snp_vp_context in the run-page union and defines stimer0 flag bits used by the SNP fast-path.
drivers/hv/mshv_vtl_main.c	Implements IPI coalescing for proxy IRR scans, adds VMSA register decode helpers, and adds an hrtimer-backed kernel fast-path for stimer0 count writes and interrupt-entry handling.

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