Re: [Xen-devel] hap_invlpg() vs INVLPGA

["Egger, Christoph" <chegger@xxxxxxxxx>]

On 29/01/16 14:24, Jan Beulich wrote:
> Christoph,
> 
> in commit dd6de3ab99 ("Implement Nested-on-Nested") you added
> code to hap_invlpg() supposedly emulating INVLPGA. I've been
> stumbling across this a number of times in the past, not being able
> to make the connection between (a) VMX/EPT and INVLPGA and
> (b) SVM's INVLPGA intercept and this function.

When you boot Windows 7 as L1 guest and XP-Mode as L2 guest then
L2 guest uses INVLPG instruction to invalidate a page and L1 guest
handles this via using INVLPGA instruction.

The INVLPG intercept flushes the nested hap p2m which is effectively
a TLB flush to the L1 guest. Then this intercept is injected into
L1 guest.

The INVLPGA instruction enforces a new ASID.

If the nested hap p2m is NULL then p2m_flush() should effectively
be a noop but it may not crash the guest.

What I don't remember is if Windows 7 must be 32bit or 64bit
to reproduce this.

Christoph

> I'm asking in the context of a reported crash resulting from the
> nv_p2m field being NULL during emulation of an INVLPG instruction
> in a guest with nesting enabled but - afaict - not actually used. Of
> course I could submit a patch adding a NULL check here, but I'd
> like to understand what this code is for, and hence whether the
> better fix wouldn't be to get rid of it.
> 
> Jan

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