Using the KVM API [LWN.net]

Using the KVM API

Posted Sep 30, 2015 23:04 UTC (Wed) by josh (subscriber, #17465)
In reply to: Using the KVM API by pbonzini
Parent article: Using the KVM API

Incidentally, with your background in qemu and kvm, perhaps you could explain (or point to documentation explaining) the purpose of KVM_SET_TSS_ADDR and KVM_SET_IDENTITY_MAP_ADDR, and what cases need them? A simple VM works without them, but the documentation lists them both as required for x86 VM setup.

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Using the KVM API

Posted Oct 1, 2015 8:21 UTC (Thu) by pbonzini (subscriber, #60935) [Link] (1 responses)

They are needed for unrestricted_guest=0. Remember that, in that case, the VM always runs in protected mode and with paging enabled. In order to emulate real mode you put the guest in a vm86 task, so you need some place for a TSS and for a page table, and they must be in guest RAM because the guest's TR and CR3 points to it. They are invisible to the guest, because the STR and MOV-from-CR instructions are invalid in vm86 mode, but it must be there.

If you don't call KVM_SET_TSS_ADDR you actually get a complaint in dmesg, and the TR stays at 0. I am not really sure what kind of bad things can happen with unrestricted_guest=0, probably you just get a VM Entry failure. The TSS takes 3 pages of memory. An interesting point is that you actually don't need to set the TR selector to a valid value (as you would do when running in "normal" vm86 mode), you can simply set the base and limit registers that are hidden in the processor, and generally inaccessible except through VMREAD/VMWRITE or system management mode. So KVM needs to set up a TSS but not a GDT.

For paging, instead, 1 page is enough because we have only 4GB of memory to address. KVM disables CR4.PAE (page address extensions, aka 8-byte entries in each page directory or page table) and enables CR4.PSE (page size extensions, aka 4MB huge pages support with 4-byte page directory entries). One page then fits 1024 4-byte page directory entries, each for a 4MB huge pages, totaling exactly 4GB. Here if you don't set it the page table is at address 0xFFFBC000. QEMU changes it to 0xFEFFC000 so that the BIOS can be up to 16MB in size (the default only allows 256k between 0xFFFC0000 and 0xFFFFFFFF).

The different handling, where only the page table has a default, is unfortunate, but so goes life...

Using the KVM API

Posted Oct 1, 2015 15:54 UTC (Thu) by josh (subscriber, #17465) [Link]

> They are needed for unrestricted_guest=0. Remember that, in that case, the VM always runs in protected mode and with paging enabled. In order to emulate real mode you put the guest in a vm86 task, so you need some place for a TSS and for a page table, and they must be in guest RAM because the guest's TR and CR3 points to it.

Ah, I see.

> If you don't call KVM_SET_TSS_ADDR you actually get a complaint in dmesg, and the TR stays at 0.

While I saw the mention of that message in a few places, I don't actually get that message at any point. Presumably that only happens with unrestricted_guest=0?

Please consider documenting the use of these two ioctls and the data they point to, as well as what circumstances require them; the current KVM documentation doesn't mention any of that.