[PATCH] KVM: x86: Expose ARCH_CAP_FB_CLEAR when invulnerable to MDS

[Jon Kohler <jon@nutanix.com>]

Expose FB_CLEAR in arch_capabilities for certain MDS-invulnerable cases 
to support live migration from older hardware (e.g., Cascade Lake, Ice 
Lake) to newer hardware (e.g., Sapphire Rapids or higher). This ensures 
compatibility when user space has previously configured vCPUs to see 
FB_CLEAR (ARCH_CAPABILITIES Bit 17).

Newer hardware sets the following bits but does not set FB_CLEAR, which 
can prevent user space from configuring a matching setup:
    ARCH_CAP_MDS_NO
    ARCH_CAP_TAA_NO
    ARCH_CAP_PSDP_NO
    ARCH_CAP_FBSDP_NO
    ARCH_CAP_SBDR_SSDP_NO

This change has minimal impact, as these bit combinations already mark 
the host as MMIO immune (via arch_cap_mmio_immune()) and set 
disable_fb_clear in vmx_update_fb_clear_dis(), resulting in no 
additional overhead.

Cc: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Jon Kohler <jon@nutanix.com>

---
 arch/x86/kvm/x86.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c841817a914a..2a4337aa78cd 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1641,6 +1641,20 @@ static u64 kvm_get_arch_capabilities(void)
 	if (!boot_cpu_has_bug(X86_BUG_GDS) || gds_ucode_mitigated())
 		data |= ARCH_CAP_GDS_NO;
 
+	/*
+	 * User space might set FB_CLEAR when starting a vCPU on a system
+	 * that does not enumerate FB_CLEAR but is also invulnerable to
+	 * other various MDS related bugs. To allow live migration from
+	 * hosts that do implement FB_CLEAR, leave it enabled.
+	 */
+	if ((data & ARCH_CAP_MDS_NO) &&
+	    (data & ARCH_CAP_TAA_NO) &&
+	    (data & ARCH_CAP_PSDP_NO) &&
+	    (data & ARCH_CAP_FBSDP_NO) &&
+	    (data & ARCH_CAP_SBDR_SSDP_NO)) {
+		data |= ARCH_CAP_FB_CLEAR;
+	}
+
 	return data;
 }
 
-- 
2.43.0