lkml.org 
[lkml]   [2021]   [Dec]   [31]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRE: [PATCH v3 22/22] kvm: x86: Disable interception for IA32_XFD on demand
Date
> From: Tian, Kevin
> Sent: Thursday, December 30, 2021 3:05 PM
>
> the new change is like below.
>
> static void handle_nm_fault_irqoff(struct kvm_vcpu *vcpu)
> {
> /*
> * Save xfd_err to guest_fpu before interrupt is enabled, so the
> * guest value is not clobbered by the host activity before the guest
> * has chance to consume it.
> *
> * Since trapping #NM is started when xfd write interception is
> * disabled, using this flag to guard the saving operation. This
> * implies no-op for a non-xfd #NM due to L1 interception.
> *
> * Queuing exception is done in vmx_handle_exit.
> */
> if (vcpu->arch.xfd_no_write_intercept)
> rdmsrl(MSR_IA32_XFD_ERR, vcpu->arch.guest_fpu.xfd_err);
> }
>
> in the final series it will first check vcpu->arch.guest_fpu.fpstate->xfd
> before the disable interception patch is applied and then becomes
> the above form, similar to your suggestion on
> vmx_update_exception_bitmap().
>
> whether to check msr_bitmap vs. an extra flag is an orthogonal open.
>
> Then:
>
> handle_exception_nmi(struct kvm_vcpu *vcpu)
> {
> ...
> if (is_machine_check(intr_info) || is_nmi(intr_info))
> return 1; /* handled by handle_exception_nmi_irqoff() */
>
> /*
> * Queue the exception here instead of in handle_nm_fault_irqoff().
> * This ensures the nested_vmx check is not skipped so vmexit can
> * be reflected to L1 (when it intercepts #NM) before reaching this
> * point.
> */
> if (is_nm_fault(intr_info)) {
> kvm_queue_exception(vcpu, NM_VECTOR);
> return 1;
> }
>
> ...
> }
>
> Then regarding to test non-AMX nested #NM usage, it might be difficult
> to trigger it from modern OS. As commented by Linux #NM handler, it's
> expected only for XFD or math emulation when fpu is missing. So we plan
> to run a selftest in L1 which sets CR0.TS and then touch fpu registers. and
> for L1 kernel we will run two binaries with one trapping #NM and the other
> not.
>

We have verified this scenario and didn't find problem.

Basically the selftest is like below:

guest_code()
{
cr0 = read_cr0();
cr0 |= X86_CR0_TS;
write_cr0(cr0);

asm volatile("fnop");
}

guest_nm_handler()
{
cr0 = read_cr0();
cr0 &= ~X86_CR0_TS;
write_cr0(cr0);
}

We run the selftest in L1 to create a nested scenario.

When L1 intercepts #NM:

(L2) fnop
(L0) #NM vmexit
(L0) reflect a virtual vmexit (reason #NM) to L1
(L1) #NM vmexit
(L1) queue #NM exception to L2
(L2) guest_nm_handler()
(L2) fnop (succeed)

When L1 doesn't intercept #NM:
(L2) fnop
(L0) #NM vmexit
(L0) queue #NM exception to L2
(L2) guest_nm_handler()
(L2) fnop (succeed)

Please suggest if any more test is necessary here.

Thanks
Kevin
\
 
 \ /
  Last update: 2021-12-31 10:43    [W:0.158 / U:0.040 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site