[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [RFC PATCH 14/35] subarch modify CPU capabilities

Chris Wright wrote:
+void __devinit machine_specific_modify_cpu_capabilities(struct cpuinfo_x86 *c)
+       clear_bit(X86_FEATURE_VME, c->x86_capability);
+       clear_bit(X86_FEATURE_DE, c->x86_capability);
+       clear_bit(X86_FEATURE_PSE, c->x86_capability);
+       clear_bit(X86_FEATURE_PGE, c->x86_capability);
+       clear_bit(X86_FEATURE_SEP, c->x86_capability);
+       clear_bit(X86_FEATURE_MWAIT, c->x86_capability);
+       if (!(xen_start_info->flags & SIF_PRIVILEGED))
+               clear_bit(X86_FEATURE_MTRR, c->x86_capability);
+       c->hlt_works_ok = 0;

That's pretty heinous. Suppose Xen decides to start supporting any of these features. Now, you need to change all of the Xen modified kernels to remove pieces of this.

You've effectively introduced completely arbitrary, but most importantly, static constraints into Linux based on what features Xen currently supports. This is not productive on either end.

This is the entire point of the VMI interface. You can arbitrarily enable and disable these features in a hidden layer, call it VMI, call it Xen-bridge, call it hypercall page, whatever. Using this layer, you can automatically abstract away exactly this type of nasty, deliberately deprecating extraneous code. We use this feature exactly to strip away these bits, completely hidden from the guest, by "trapping" the CPUID instruction in the VMI layer. The VMI interface as it stands today is unimportant - it has and will continue to change to accommodate Xen. But the principles of it are important for maintainability, flexibility, and future compatibility. One of these principles is to avoid unbalanced changes like this to the guest kernel.

And we certainly support VME, DE, PSE, PGE, and SEP features, and really would not like them disabled unconditionally in a virtual environment.


Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.