[PATCH v8 08/21] KVM: VMX: Save/restore guest FRED RSP0

Xin Li (Intel)
2025-10-14T01:11:13Z

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 <xin3.li@intel.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Tested-by: Shan Kang <shan.kang@intel.com>
Tested-by: Xuelian Guo <xuelian.guo@intel.com>
---

Changes in v5:
* Remove the cpu_feature_enabled() check when set/get guest
MSR_IA32_FRED_RSP0, as guest_cpu_cap_has() should suffice (Sean).
* Add a comment when synchronizing current MSR_IA32_FRED_RSP0 MSR to
the kernel's local cache, because its handling is different from
the MSR_KERNEL_GS_BASE handling (Sean).
* Add TB from Xuelian Guo.

Changes in v3:
* KVM only needs to save/restore guest FRED RSP0 now as host FRED RSP0
is restored in arch_exit_to_user_mode_prepare() (Sean Christopherson).

Changes in v2:
* Don't use guest_cpuid_has() in vmx_prepare_switch_to_{host,guest}(),
which are called from IRQ-disabled context (Chao Gao).
* Reset msr_guest_fred_rsp0 in __vmx_vcpu_reset() (Chao Gao).
---
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 cced21832ee6..3ea660c0481c 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1292,6 +1292,9 @@ 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);
@@ -1336,6 +1339,16 @@ 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 2cf599211ab3..c4a3b28553fb 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -227,6 +227,7 @@ 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;
--
2.51.0