Re: [Xen-devel] Nested virtualization off VMware vSphere 6.0 with EL6 guests crashes on Xen 4.6

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.01.16 at 22:39, <konrad.wilk@xxxxxxxxxx> wrote:
> On Tue, Jan 12, 2016 at 02:22:03AM -0700, Jan Beulich wrote:
>> Since we can (I hope) pretty much exclude a paging type, the
>> ASSERT() must have triggered because of vapic_pg being NULL.
>> That might be verifiable without extra printk()s, just by checking
>> the disassembly (assuming the value sits in a register). In which
>> case vapic_gpfn would be of interest too.
> 
> The vapic_gpfn is 0xffffffffffff.
> 
> To be exact:
> 
> nvmx_update_virtual_apic_address:vCPU0 0xffffffffffffffff(vAPIC) 0x0(APIC), 
> 0x0(TPR) ctrl=b5b9effe
> 
> Based on this:
> 
> diff --git a/xen/arch/x86/hvm/vmx/vvmx.c b/xen/arch/x86/hvm/vmx/vvmx.c
> index cb6f9b8..8a0abfc 100644
> --- a/xen/arch/x86/hvm/vmx/vvmx.c
> +++ b/xen/arch/x86/hvm/vmx/vvmx.c
> @@ -695,7 +695,15 @@ static void nvmx_update_virtual_apic_address(struct vcpu 
> *v)
>  
>          vapic_gpfn = __get_vvmcs(nvcpu->nv_vvmcx, VIRTUAL_APIC_PAGE_ADDR) >> 
> PAGE_SHIFT;
>          vapic_pg = get_page_from_gfn(v->domain, vapic_gpfn, &p2mt, 
> P2M_ALLOC);
> -        ASSERT(vapic_pg && !p2m_is_paging(p2mt));
> +       if ( !vapic_pg ) {
> +               printk("%s:vCPU%d 0x%lx(vAPIC) 0x%lx(APIC), 0x%lx(TPR) 
> ctrl=%x\n", __func__,v->vcpu_id,
> +                       __get_vvmcs(nvcpu->nv_vvmcx, VIRTUAL_APIC_PAGE_ADDR),
> +                       __get_vvmcs(nvcpu->nv_vvmcx, APIC_ACCESS_ADDR),
> +                       __get_vvmcs(nvcpu->nv_vvmcx, TPR_THRESHOLD),
> +                       ctrl);
> +       }
> +        ASSERT(vapic_pg);
> +       ASSERT(vapic_pg && !p2m_is_paging(p2mt));
>          __vmwrite(VIRTUAL_APIC_PAGE_ADDR, page_to_maddr(vapic_pg));
>          put_page(vapic_pg);
>      }

Interesting: I can't see VIRTUAL_APIC_PAGE_ADDR to be written
with all ones anywhere, neither for the real VMCS nor for the virtual
one (page_to_maddr() can't, afaict, return such a value). Could you
check where the L1 guest itself is writing that value, or whether it
fails to initialize that field and it happens to start out as all ones?

>> What looks odd to me is the connection between
>> CPU_BASED_TPR_SHADOW being set and the use of a (valid)
>> virtual APIC page: Wouldn't this rather need to depend on
>> SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES, just like in
>> nvmx_update_apic_access_address()?
> 
> Could be. I added in an read for the secondary control:
> 
> nvmx_update_virtual_apic_address:vCPU2 0xffffffffffffffff(vAPIC) 0x0(APIC), 
> 0x0(TPR) ctrl=b5b9effe sec=0
> 
> So trying your recommendation:
> diff --git a/xen/arch/x86/hvm/vmx/vvmx.c b/xen/arch/x86/hvm/vmx/vvmx.c
> index cb6f9b8..d291c91 100644
> --- a/xen/arch/x86/hvm/vmx/vvmx.c
> +++ b/xen/arch/x86/hvm/vmx/vvmx.c
> @@ -686,8 +686,8 @@ static void nvmx_update_virtual_apic_address(struct vcpu 
> *v)
>      struct nestedvcpu *nvcpu = &vcpu_nestedhvm(v);
>      u32 ctrl;
>  
> -    ctrl = __n2_exec_control(v);
> -    if ( ctrl & CPU_BASED_TPR_SHADOW )
> +    ctrl = __n2_secondary_exec_control(v);
> +    if ( ctrl & SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES )
>      {
>          p2m_type_t p2mt;
>          unsigned long vapic_gpfn;
> 
> 
> Got me:
> (XEN) stdvga.c:151:d1v0 leaving stdvga mode
> (XEN) stdvga.c:147:d1v0 entering stdvga and caching modes
> (XEN) stdvga.c:520:d1v0 leaving caching mode
> (XEN) vvmx.c:2491:d1v0 Unknown nested vmexit reason 80000021.
> (XEN) Failed vm entry (exit reason 0x80000021) caused by invalid guest state 

Interesting. I've just noticed that a similar odd looking (to me)
dependency exists in construct_vmcs(). Perhaps I've overlooked
something in the SDM. In any event I think some words from the
VMX maintainers would be quite nice here.

Sadly the VMCS dump doesn't include the two APIC related
addresses...

Jan


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