[PATCH v10 09/28] KVM: VMX: Save/restore guest FRED RSP0

[Sohil Mehta <sohil.mehta@intel.com>]

From: "Xin Li (Intel)" <xin@zytor.com>

Save guest FRED RSP0 in vmx_prepare_switch_to_host() and restore it
in vmx_prepare_switch_to_guest() because MSR_IA32_FRED_RSP0 is passed
through to the guest, thus is volatile/unknown.

Note, host FRED RSP0 is restored in arch_exit_to_user_mode_prepare(),
regardless of whether it is modified in KVM.

Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
---
v10:
 - No change

Note, an equivalent TDX side implementation is missing. No TDX module
with FRED support exists yet so this would be an issue for future
platforms. A generic discussion regarding this is ongoing at:
https://lore.kernel.org/kvm/20260827031837.2863609-1-binbin.wu@linux.intel.com/
---
 arch/x86/kvm/vmx/vmx.c | 13 +++++++++++++
 arch/x86/kvm/vmx/vmx.h |  1 +
 2 files changed, 14 insertions(+)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 09bbbc680aa7..b6b73ab9cfb7 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1395,6 +1395,9 @@ void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
 	}
 
 	wrmsrq(MSR_KERNEL_GS_BASE, vmx->msr_guest_kernel_gs_base);
+
+	if (guest_cpu_cap_has(vcpu, X86_FEATURE_FRED))
+		wrmsrns(MSR_IA32_FRED_RSP0, vmx->msr_guest_fred_rsp0);
 #else
 	savesegment(fs, fs_sel);
 	savesegment(gs, gs_sel);
@@ -1439,6 +1442,16 @@ static void vmx_prepare_switch_to_host(struct vcpu_vmx *vmx)
 	invalidate_tss_limit();
 #ifdef CONFIG_X86_64
 	wrmsrq(MSR_KERNEL_GS_BASE, vmx->vt.msr_host_kernel_gs_base);
+
+	if (guest_cpu_cap_has(&vmx->vcpu, X86_FEATURE_FRED)) {
+		vmx->msr_guest_fred_rsp0 = read_msr(MSR_IA32_FRED_RSP0);
+		/*
+		 * Synchronize the current value in hardware to the kernel's
+		 * local cache.  The desired host RSP0 will be set when the
+		 * CPU exits to userspace (RSP0 is a per-task value).
+		 */
+		fred_sync_rsp0(vmx->msr_guest_fred_rsp0);
+	}
 #endif
 	load_fixmap_gdt(raw_smp_processor_id());
 	vmx->vt.guest_state_loaded = false;
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index 7a22e1b1a273..14fd3f57d5c1 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -222,6 +222,7 @@ struct vcpu_vmx {
 	bool                  guest_uret_msrs_loaded;
 #ifdef CONFIG_X86_64
 	u64		      msr_guest_kernel_gs_base;
+	u64		      msr_guest_fred_rsp0;
 #endif
 
 	u64		      spec_ctrl;
-- 
2.43.0