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Re: [Xen-devel] [PATCH 5/9] x86/vmx: Fix handing of MSR_DEBUGCTL on VMExit
- To: "Tian, Kevin" <kevin.tian@xxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 28 May 2018 16:30:18 +0100
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- Cc: Wei Liu <wei.liu2@xxxxxxxxxx>, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Mon, 28 May 2018 15:30:33 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 27/05/18 04:56, Tian, Kevin wrote:
>> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
>> Sent: Tuesday, May 22, 2018 7:21 PM
>>
>> Currently, whenever the guest writes a nonzero value to MSR_DEBUGCTL,
>> Xen
>> updates a host MSR load list entry with the current hardware value of
>> MSR_DEBUGCTL. This is wrong.
>>
>> On VMExit, hardware automatically resets MSR_DEBUGCTL to 0. The only
>> case
>> where different behaviour is needed is if Xen is debugging itself, and this
>> needs setting up unconditionally for the lifetime of the VM.
>>
>> The `ler` command line boolean is the only way to configure any use of
>> MSR_DEBUGCTL for Xen, so tie the host load list entry to this setting in
>> construct_vmcs(). Any runtime update of Xen's MSR_DEBUGCTL setting
>> requires
>> more complicated synchronisation across all the running VMs.
>>
>> In the exceedingly common case, this avoids the unnecessary overhead of
>> having
>> a host load entry performing the same zeroing operation that hardware
>> has
>> already performed as part of the VMExit.
> I didn't get "unnecessary overhead" part. if "ler' is disabled, as you
> said earlier it's a bug to save/restore thus overhead doesn't matter.
The current behaviour (bug or otherwise), means that when ler is
disabled, we end up with a host load list entry zeroing MSR_DEBUGCTL as
soon as the guest first writes to the MSR.
This causes unnecessary overhead because host load/save lists are slow.
Irrespective, I've reworked this patch differently, to fully disentangle
it from the EFER series. See patch 1 of my "x86/vmx: Misc fixes and
improvements" series.
~Andrew
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