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Re: [Xen-devel] [PATCH] x86/msr: Fix fallout from mostly c/s 832c180
- To: Wei Liu <wei.liu2@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 10 Apr 2019 10:41:24 +0100
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- Cc: Kevin Tian <kevin.tian@xxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>, Paul Durrant <paul.durrant@xxxxxxxxxx>, Jun Nakajima <jun.nakajima@xxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 10 Apr 2019 09:41:35 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 10/04/2019 09:23, Wei Liu wrote:
> On Tue, Apr 09, 2019 at 06:53:47PM +0100, Andrew Cooper wrote:
>> The series 832c1803^..f61685a6 was committed without adequate review.
>>
>> * Fix the shim build by providing a !CONFIG_HVM declaration for
>> hvm_get_guest_bndcfgs()
>> * Revert the bogus de-const'ing of the vcpu pointer in
>> vmx_get_guest_bndcfgs(). vmx_vmcs_enter() really does mutate the vcpu,
>> and
>> may cause it to undergo a full de/reschedule, which is in violation of the
>> ABI described by hvm_get_guest_bndcfgs(). guest_rdmsr() was always going
>> to need to lose its const parameter, and this was the correct time for it
>> to happen.
> Enlighten me -- why does guest_rdmsr need to lose its const parameter?
>
> This reads as if the code was buggy from the get-go.
It was originally written without a const vcpu *, in full knowledge that
when moving onto some of the less trivial MSRs, it would need a mutable
parameter.
Of course, this wasn't considered a good enough argument during review,
and to avoid an argument, it went in with a const parameter.
The state of the MSRs may live in various memory structures, or directly
in hardware (covers the two cases thus far), or in the VMCS/VMCB, (or
worse, the MSR load/save lists). This patch adds the first example of
needing to access data straight from the VMCS, which involves
negotiating with the scheduler for safe access.
~Andrew
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