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Re: [RFC PATCH 03/22] x86/msr: always allow a pinned Dom0 to read any unknown MSR
- To: Edwin Török <edwin.torok@xxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 30 Oct 2023 17:29:09 +0100
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- Cc: Edwin Török <edvin.torok@xxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Jun Nakajima <jun.nakajima@xxxxxxxxx>, Kevin Tian <kevin.tian@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 30 Oct 2023 16:29:24 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 25.10.2023 21:29, Edwin Török wrote:
> This can be useful if you realize you have to inspect the value of an
> MSR in production, without having to change into a new Xen first that
> handles the MSR.
Yet on a non-pinned Dom0 you'd still be lost. Since iirc we generally
advise against pinning, I wonder of how much use such a change would be,
when it effectively undoes what we deliberately did a while ago.
> --- a/xen/arch/x86/hvm/svm/svm.c
> +++ b/xen/arch/x86/hvm/svm/svm.c
> @@ -1933,6 +1933,9 @@ static int cf_check svm_msr_read_intercept(
> break;
>
> default:
> + if ( is_hwdom_pinned_vcpu(v) && !rdmsr_safe(msr, *msr_content) )
> + break;
> +
> if ( d->arch.msr_relaxed && !rdmsr_safe(msr, tmp) )
> {
> *msr_content = 0;
If we went as far as undoing some of what was done, I'd then wonder
whether instead we should mandate relaxed mode to be enabled on such a
Dom0. Then, instead of returning fake 0 here, the actual value could
be returned in the specific case of (pinned?) Dom0.
Jan
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