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

Re: [Xen-devel] [PATCH 6/6] x86/emul: dedup hvmemul_cpuid() and pv_emul_cpuid()


  • To: Jan Beulich <JBeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Fri, 9 Nov 2018 17:16:58 +0000
  • Autocrypt: addr=andrew.cooper3@xxxxxxxxxx; prefer-encrypt=mutual; keydata= xsFNBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABzSlBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPsLBegQTAQgAJAIbAwULCQgHAwUVCgkI CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt TQTBLzDKXok86M7BTQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAcLB XwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA==
  • Cc: Wei Liu <wei.liu2@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>
  • Delivery-date: Fri, 09 Nov 2018 17:17:09 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 06/11/18 16:16, Jan Beulich wrote:
>>>> On 06.11.18 at 16:52, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 06/11/18 15:38, Jan Beulich wrote:
>>>>>> On 05.11.18 at 12:21, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> They are identical, so provide a single x86emul_cpuid() instead.
>>>>
>>>> As x86_emulate() now only uses the ->cpuid() hook for real CPUID 
>> instructions,
>>>> the hook can be omitted from all special-purpose emulation ops.
>>> So I was expecting the hook to go away altogether, but I
>>> now realize that it can't because of some of the customization
>>> that's needed. That, in turn, means that the removal of the
>>> hook specification as per above will get us into problems the
>>> moment we need to check a feature that can't be taken
>>> straight from the policy object. I'm therefore unconvinced we
>>> actually want to go this far. It'll require enough care already
>>> to not blindly clone a new vcpu_has_...() wrongly from the
>>> many pre-existing examples in such a case. Thoughts?
>> All dynamic bits in CPUID are derived from other control state.  e.g. we
>> check CR4.OSXSAVE, not CPUID.OSXSAVE.  The other dynamic bits are APIC,
>> which comes from MSR_APIC_BASE, and OSPKE which also comes from CR4.
>>
>> In the emulator itself, I think it would be a bug if we ever had
>> vcpu_has_osxsave etc, because that isn't how pipelines actually behave. 
>> The feature checks here are semantically equivalent to "do the
>> instruction decode and execution units have silicon to cope with these
>> instructions".
> I agree that vcpu_has_os* makes no sense, but the APIC bit for
> example isn't really pipeline / decoder related.

Correct, so why do you think APIC matters?  All interaction with the
APIC is via its MMIO/MSR interface, rather than via dedicated instructions.

> However, one
> issue already might be that in order for bits in a (sub)leaf above
> (guest) limits to come out all clear, it is guest_cpuid() which cuts
> things off. Neither cpuid_featureset_to_policy() nor its inverse
> nor sanitise_featureset() look to zap fields above limits, in case
> they've been previously set to non-zero values. Am I overlooking
> something?

No - that is an aspect I'd overlooked, because the
DOMCTL_set_cpuid_policy work (which does this correctly) hasn't gone in yet.

I think I'll tweak recalculate_misc() to zero out beyond the max_subleaf
settings, because the intention was always that a flat cpuid_policy was
safe to use in this manner.  I think there is an existing corner case
when trying to level basic.max_leaf to < 7, or extd.max_leaf to < 0x8000007.

> Furthermore I wouldn't exclude that we may need to look at a
> hypervisor or Viridian leaf at some point. The dynamic vPMU
> adjustments also look potentially problematic, if we were to
> emulate RDPMC (albeit they're DS-related only right now).

The only reason vPMU is dynamic is because we don't (yet) have a split
between default and max policies.  Fixing this is on the todo list,
albeit behind a fairly long chain of dependencies.

> And then there's the dynamic exposing of MONITOR for PV; granted
> I don't expect us to ever emulate this for PV, but it shows the
> general issue.

That is to work around a deficiency in how Linux behaves.  It isn't for
allowing PV guests to use MONITOR.

> Plus there's SYSCALL, the insn emulation of which
> currently doesn't check the (dynamically adjusted) CPUID bit.

No, nor should it.  Intel's objection to the SYSCALL/SYSRET instructions
is mode based.  (As a demonstration which proves the point of this
patch, if you hide the SYSCALL bit using masking, the instruction still
operates fine).

It seems I never got around to submitting my XSA-204 followup patch
which fixes many emulation bugs with SYS{CALL,RET,ENTER,EXIT}.

> And finally LWP, which again we can only hope we'll never have
> to emulate.

LWP doesn't exist any more, even on the hardware it used to exist on. 
It was never implemented on Fam17h, and was removed from Fam15/16h in a
microcode update to make room to implement IBPB for Spectre v2 mitigations.

I recommend we purge the support completely.

~Andrew

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel

 


Rackspace

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