Re: [Xen-devel] [PATCH 08/11] x86/altp2m: add remaining support routines.

[Ed White <edmund.h.white@xxxxxxxxx>]

On 01/15/2015 09:25 AM, Tim Deegan wrote:
> Hi,
> 
> At 13:26 -0800 on 09 Jan (1420806398), Ed White wrote:
>> +int
>> +altp2mhvm_hap_nested_page_fault(struct vcpu *v, paddr_t gpa,
>> +                                unsigned long gla, struct npfec npfec)
>> +{
>> +    struct domain *d = v->domain;
>> +    struct p2m_domain *hp2m = p2m_get_hostp2m(d);
>> +    struct p2m_domain *ap2m;
>> +    p2m_type_t p2mt;
>> +    p2m_access_t p2ma;
>> +    unsigned int page_order;
>> +    unsigned long gfn, mask;
>> +    mfn_t mfn;
>> +    int rv;
>> +
>> +    ap2m = p2m_get_altp2m(v);
>> +
>> +    mfn = get_gfn_type_access(ap2m, gpa >> PAGE_SHIFT, &p2mt, &p2ma,
>> +                              0, &page_order);
>> +    __put_gfn(ap2m, gpa >> PAGE_SHIFT);
>> +
>> +    if ( mfn_valid(mfn) )
>> +    {
>> +        /* Should #VE be emulated for this fault? */
>> +        if ( p2mt == p2m_ram_rw_ve && !cpu_has_vmx_virt_exceptions &&
>> +             ahvm_vcpu_emulate_ve(v) )
>> +            return ALTP2MHVM_PAGEFAULT_DONE;
>> +
>> +        /* Could not handle fault here */
>> +        gdprintk(XENLOG_INFO, "Altp2m memory access permissions failure, "
>> +                              "no mem_event listener VCPU %d, dom %d\n",
>> +                              v->vcpu_id, d->domain_id);
>> +        domain_crash(v->domain);
>> +        return ALTP2MHVM_PAGEFAULT_CONTINUE;
>> +    }
>> +
>> +    mfn = get_gfn_type_access(hp2m, gpa >> PAGE_SHIFT, &p2mt, &p2ma,
>> +                              0, &page_order);
>> +    put_gfn(hp2m->domain, gpa >> PAGE_SHIFT);
>> +
>> +    if ( p2mt != p2m_ram_rw || p2ma != p2m_access_rwx )
>> +        return ALTP2MHVM_PAGEFAULT_CONTINUE;
> 
> I don't follow -- surely the altp2m ought to contain everything that's
> in the host p2m except for deliberate extra changes.  But it looks
> like here you just bail on anything other than rwx RAM.  That's going
> to livelock on anything that the main fault handler thinks is OK to retry.

It sounds like there must be some cases that I'm not aware of.

I've only tested Windows, and I've never seen anything other than rwx ram
where the hardware is ever able to retry and succeed. That's why I don't
copy anything else to the alternate p2m, because my experience has been
that everything else can be resolved in the host p2m and its nested page
fault handler. As I explained to Jan, if the host p2m page becomes rwx ram
and *then* there's a retry, I copy that EPTE on the violation that the retry
triggers and retry again.

I want to get this right. Can you tell me what I'm missing?

Ed


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