CVE-2025-38508: x86/sev: Use TSC_FACTOR for Secure TSC frequency calculation

In the Linux kernel, the following vulnerability has been resolved:

x86/sev: Use TSCFACTOR for Secure TSC frequency calculation

When using Secure TSC, the GUESTTSCFREQ MSR reports a frequency based on the nominal P0 frequency, which deviates slightly (typically ~0.2%) from the actual mean TSC frequency due to clocking parameters.

Over extended VM uptime, this discrepancy accumulates, causing clock skew between the hypervisor and a SEV-SNP VM, leading to early timer interrupts as perceived by the guest.

The guest kernel relies on the reported nominal frequency for TSC-based timekeeping, while the actual frequency set during SNPLAUNCHSTART may differ. This mismatch results in inaccurate time calculations, causing the guest to perceive hrtimers as firing earlier than expected.

Utilize the TSCFACTOR from the SEV firmware's secrets page (see "Secrets Page Format" in the SNP Firmware ABI Specification) to calculate the mean TSC frequency, ensuring accurate timekeeping and mitigating clock skew in SEV-SNP VMs.

Use earlyioremapencrypted() to map the secrets page as ioremapencrypted() uses kmalloc() which is not available during early TSC initialization and causes a panic.

[ bp: Drop the silly dummy var: https://lore.kernel.org/r/20250630192726.GBaGLlHl84xIopx4Pt@fatcrate.local ]