Re: [Xen-devel] [PATCH v4] x86/HVM: make hvm_efer_valid() honor guest features

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.01.15 at 21:08, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 19/01/15 15:12, Jan Beulich wrote:
>> Following the earlier similar change validating CR4 modifications.
>>
>> Note that this requires a non-obvious adjustment to hvm_cpuid(): On
>> 32-bit hosts/tool stacks, the SYSCALL feature flag will be seen clear
>> on Intel CPUs, yet 64-bit guests legitimately expect the feature for
>> be available. Mimic hardware behavior: When executed from a 64-bit
>> code segment, force the feature flag set.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> I finally have the debugging out of XenRT as to why this is still
> failing for windows (including my improvements to the error message
> which I will post in due course).
> 
> d58v1 Invalid EFER update: 0 -> 0x901 - SCE without feature
> 
> When windows brings up its second cpu, it appears to set up EFER
> completely in one go, which means it will still be in protected mode at
> this point.  It sets SCE in the knowledge that the cpu is 64bit, and
> this indicates that it is valid to set EFER.SCE on 64-bit Intel cpus
> even the feature would not appear via cpuid.
> 
> Undoing the v3 -> v4 change wrt hvm_cpuid() fixes the issue, and I rerun
> the test suite like this.

Now that's odd - v4 only changes _where_ the SYSCALL feature
flag gets set, not the dependency on CS.L. Or are you saying
that this vCPU hotplug issue is independent of the 32-bit tool
stack one that v3 fixed, and hence it is a problem that the VMX
code actively clears the feature flag?

> I suggest that the easiest course of action is to simply ignore this
> Intelism wrt EFER.SCE, and add it to the big bucket of edge cases that
> we can't quite virtualise correctly.
> 
> On the other hand, I wonder whether it is permitted because LME is set
> at the same time?  It might be that SCE is permitted if and only if LME
> has been verified as ok.

And I think I'll go that route - permit SCE when the feature flag is
set, or would be set after entering long mode when enabling long
mode at the same time.

Jan


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