Re: [Xen-devel] Getting the XSAVE size from userspace

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 11/05/2015 01:44 PM, Andrew Cooper wrote:
> On 05/11/15 11:35, Andrei LUTAS wrote:
>> The use-case is the following: whenever an EPT violation is triggered
>> inside a monitored VM, the introspection logic needs to know how many
>> bytes were accessed (read/written). This is done by inspecting the
>> faulting instruction and directly inferring the size, which is not
>> straight-forward for XSAVE/XRSTOR family. Using the maximum possible
>> size is wrong, as in any given moment the OS may or may not desire to
>> XSAVE/XRSTOR the entire state (and thinking that the instruction tries
>> to access more than it actually does may yield undesired effects).
>> Therefore, the size needed for the currently enabled features of the
>> monitored guest is required instead. Normally, it could be done by
>> running CPUID with eax = 0xD and ecx = i, where i >= 2 and XCR0[i] is
>> 1 (XCR0 belongs to the monitored guest), but I am unsure if using
>> CPUID this way would be safe/desired: will Xen expose the same CPUID
>> features, for XSAVE related functionality, on all VMs? (using XCPUID
>> with eax = 0xD and ecx = 0 would give us the needed size for the SVA,
>> and like I said, using the maximum size would not be safe, even if
>> it's the same across all VMs on a given host). Also, I'm unsure how
>> this would get along with migration...
> 
> Hmm yes - there is no way to do this currently.
> 
> Xen's CPUID handling for xsave related things is broken in levelling and
> migration scenarios, which is why it is *still* disabled by default in
> XenServer.
> 
> I am working on fixing it, and will take this usecase into account
> (although I think I had already included enough for this usecase to work).
> 
> At the point of the xsave/xrestor trap, you need to know xcr0 and be
> able to perfom a cpuid instruction in the context of a target domain, to
> make use of 0xD[0].ebx to get the "current size based on xcr0".

So then the closest thing to what we need would be to add a size field
to struct hvm_hw_cpu_xsave, and just assign the size variable to it in
hvm_save_cpu_xsave_states (migration aside)?

2130 static int hvm_save_cpu_xsave_states(struct domain *d,
hvm_domain_context_t *h)
2131 {
2132     struct vcpu *v;
2133     struct hvm_hw_cpu_xsave *ctxt;
2134
2135     if ( !cpu_has_xsave )
2136         return 0;   /* do nothing */
2137
2138     for_each_vcpu ( d, v )
2139     {
2140         unsigned int size = HVM_CPU_XSAVE_SIZE(v->arch.xcr0_accum);
2141
2142         if ( !xsave_enabled(v) )
2143             continue;
2144         if ( _hvm_init_entry(h, CPU_XSAVE_CODE, v->vcpu_id, size) )
2145             return 1;
2146         ctxt = (struct hvm_hw_cpu_xsave *)&h->data[h->cur];
2147         h->cur += size;
2148
2149         ctxt->xfeature_mask = xfeature_mask;
2150         ctxt->xcr0 = v->arch.xcr0;
2151         ctxt->xcr0_accum = v->arch.xcr0_accum;
2152         memcpy(&ctxt->save_area, v->arch.xsave_area,
2153                size - offsetof(struct hvm_hw_cpu_xsave, save_area));
2154     }
2155
2156     return 0;
2157 }


Thanks,
Razvan

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