[RFC PATCH 40/73] KVM: x86/PVM: Handle hypercall for loading GS selector

Lai Jiangshan
2024-02-26T14:36:48Z

SWAPGS is not supported in PVM, so the native load_gs_index() cannot be
used in the guest. Therefore, a hypercall is introduced to load the GS
selector into the GS segment register, and the resulting GS base is
returned to the guest. This is prepared for supporting 32-bit processes.

Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
---
arch/x86/kvm/pvm/pvm.c | 71 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 71 insertions(+)

diff --git a/arch/x86/kvm/pvm/pvm.c b/arch/x86/kvm/pvm/pvm.c
index ad08643c098a..ee55e99fb204 100644
--- a/arch/x86/kvm/pvm/pvm.c
+++ b/arch/x86/kvm/pvm/pvm.c
@@ -1528,6 +1528,75 @@ static int handle_hc_invlpg(struct kvm_vcpu *vcpu, unsigned long addr)
return 1;
}

+/*
+ * Hypercall: PVM_HC_LOAD_GS
+ * Load %gs with the selector %rdi and load the resulted base address
+ * into RAX.
+ *
+ * If %rdi is an invalid selector (including RPL != 3), NULL selector
+ * will be used instead.
+ *
+ * Return the resulted GS BASE in vCPU's RAX.
+ */
+static int handle_hc_load_gs(struct kvm_vcpu *vcpu, unsigned short sel)
+{
+ struct vcpu_pvm *pvm = to_pvm(vcpu);
+ unsigned long guest_kernel_gs_base;
+
+ /* Use NULL selector if RPL != 3. */
+ if (sel != 0 && (sel & 3) != 3)
+ sel = 0;
+
+ /* Protect the guest state on the hardware. */
+ preempt_disable();
+
+ /*
+ * Switch to the guest state because the CPU is going to set the %gs to
+ * the guest value. Save the original guest MSR_GS_BASE if it is
+ * already the guest state.
+ */
+ if (!pvm->loaded_cpu_state)
+ pvm_prepare_switch_to_guest(vcpu);
+ else
+ __save_gs_base(pvm);
+
+ /*
+ * Load sel into %gs, which also changes the hardware MSR_KERNEL_GS_BASE.
+ *
+ * Before load_gs_index(sel):
+ * hardware %gs: old gs index
+ * hardware MSR_KERNEL_GS_BASE: guest MSR_GS_BASE
+ *
+ * After load_gs_index(sel);
+ * hardware %gs: resulted %gs, @sel or NULL
+ * hardware MSR_KERNEL_GS_BASE: resulted GS BASE
+ *
+ * The resulted %gs is the new guest %gs and will be saved into
+ * pvm->segments[VCPU_SREG_GS].selector later when the CPU is
+ * switching to host or the guest %gs is read (pvm_get_segment()).
+ *
+ * The resulted hardware MSR_KERNEL_GS_BASE will be returned via RAX
+ * to the guest and the hardware MSR_KERNEL_GS_BASE, which represents
+ * the guest MSR_GS_BASE when in VM-Exit state, is restored back to
+ * the guest MSR_GS_BASE.
+ */
+ load_gs_index(sel);
+
+ /* Get the resulted guest MSR_KERNEL_GS_BASE. */
+ rdmsrl(MSR_KERNEL_GS_BASE, guest_kernel_gs_base);
+
+ /* Restore the guest MSR_GS_BASE into the hardware MSR_KERNEL_GS_BASE. */
+ __load_gs_base(pvm);
+
+ /* Finished access to the guest state on the hardware. */
+ preempt_enable();
+
+ /* Return RAX with the resulted GS BASE. */
+ kvm_rax_write(vcpu, guest_kernel_gs_base);
+
+ return 1;
+}
+
/*
* Hypercall: PVM_HC_RDMSR
* Write MSR.
@@ -1604,6 +1673,8 @@ static int handle_exit_syscall(struct kvm_vcpu *vcpu)
return handle_hc_flush_tlb_current_kernel_user(vcpu);
case PVM_HC_TLB_INVLPG:
return handle_hc_invlpg(vcpu, a0);
+ case PVM_HC_LOAD_GS:
+ return handle_hc_load_gs(vcpu, a0);
case PVM_HC_RDMSR:
return handle_hc_rdmsr(vcpu, a0);
case PVM_HC_WRMSR:
--
2.19.1.6.gb485710b