Re: [Xen-devel] [PATCH v2 7/9] x86/vmx: Support load-only guest MSR list entries

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Sent: Saturday, June 9, 2018 2:49 AM
> 
> Currently, the VMX_MSR_GUEST type maintains completely symmetric
> guest load
> and save lists, by pointing VM_EXIT_MSR_STORE_ADDR and
> VM_ENTRY_MSR_LOAD_ADDR
> at the same page, and setting VM_EXIT_MSR_STORE_COUNT and
> VM_ENTRY_MSR_LOAD_COUNT to the same value.
> 
> However, for MSRs which we won't let the guest have direct access to,
> having
> hardware save the current value on VMExit is unnecessary overhead.
> 
> To avoid this overhead, we must make the load and save lists asymmetric.
> By
> making the entry load count greater than the exit store count, we can
> maintain
> two adjacent lists of MSRs, the first of which is saved and restored, and the
> second of which is only restored on VMEntry.
> 
> For simplicity:
>  * Both adjacent lists are still sorted by MSR index.
>  * It undefined behaviour to insert the same MSR into both lists.
>  * The total size of both lists is still limited at 256 entries (one 4k page).
> 
> Split the current msr_count field into msr_{load,save}_count, and introduce
> a
> new VMX_MSR_GUEST_LOADONLY type, and update vmx_{add,find}_msr()
> to calculate
> which sublist to search, based on type.  VMX_MSR_HOST has no logical
> sublist,
> whereas VMX_MSR_GUEST has a sublist between 0 and the save count,
> while
> VMX_MSR_GUEST_LOADONLY has a sublist between the save count and
> the load
> count.
> 
> One subtle point is that inserting an MSR into the load-save list involves
> moving the entire load-only list, and updating both counts.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Acked-by: Kevin Tian <kevin.tian@xxxxxxxxx>
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