[Xen-devel] Ping VT-x: [PATCH 10/11] x86/vmx: Work around VMEntry failure when Single Stepping in an STI shadow

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 04/06/18 14:59, Andrew Cooper wrote:
> See the code comment for the details.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> CC: Kevin Tian <kevin.tian@xxxxxxxxx>
>
> Jun/Kevin: This workaround is as suggested by Gil, and there is expected to be
> an SDM update discussing the corner case.
>
> Note that, like elsewhere dealing with eflags.tf, this is probably buggy in
> combination with MSR_DEBUGCTL.BTF.  I'll untangle the BTF swamp at some later
> point.
> ---
>  xen/arch/x86/hvm/vmx/vmx.c | 30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
>
> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> index 82ef3aa..58ff8c7 100644
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -1794,6 +1794,36 @@ static void vmx_inject_event(const struct x86_event 
> *event)
>          write_debugreg(6, merge_dr6(read_debugreg(6), event->pending_dbg,
>                                      curr->domain->arch.cpuid->feat.rtm));
>  
> +        /*
> +         * Work around SS/STI vmentry bug.
> +         *
> +         * If kernel code is single stepping itself and executes an STI
> +         * instruction resulting in an STI shadow, a vmexit occurs due to #DB
> +         * interception, but the vmentry fails due to a failed consistency
> +         * check.  (Hardware comes to the conclusion that there should be a
> +         * pending debug exception, but doesn't account for the pending #DB 
> in
> +         * VMENTRY_INTR_INFO.)
> +         *
> +         * Manually adjust the pending debug exception field to mark BS as
> +         * pending, which satisfies the consistency check and allows the
> +         * vmentry to succeed.
> +         */
> +        if ( unlikely(regs->eflags & X86_EFLAGS_TF) )
> +        {
> +            unsigned long int_info;
> +
> +            __vmread(GUEST_INTERRUPTIBILITY_INFO, &int_info);
> +
> +            if ( int_info & VMX_INTR_SHADOW_STI )
> +            {
> +                unsigned long pending_dbg;
> +
> +                __vmread(GUEST_PENDING_DBG_EXCEPTIONS, &pending_dbg);
> +                __vmwrite(GUEST_PENDING_DBG_EXCEPTIONS,
> +                          pending_dbg | X86_DR6_BS);
> +            }
> +        }
> +
>          if ( !nestedhvm_vcpu_in_guestmode(curr) ||
>               !nvmx_intercepts_exception(curr, TRAP_debug, _event.error_code) 
> )
>          {


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