>Sent: Tuesday, May 31, 2011 12:16 AM
could you use plain text format when posting mail next time, which is easier
for reply? :-)
> Question might be simple but really confused me a long time.
> In my assumption, during VM running, the CPU sees the guest code instruction,
>so when it want to asscess memory, it sees the guest virtual adress(GVA), so
>it need to
>first translate to guest physical adress(GPA), and then host physical
>adress(HPA), and finally
>access the real memory address.
>Since GPA are always needs to translated to HPA, and p2m table only accessable
>in Xen for
>HVM guest, does this mean every memory access will need VMExit to Xen?
W/o EPT, CPU only has VA->PA knowledge, and thus here comes the concept of
shadow page table. Xen marks guest page table as read-only, and then tracks
every update to guest page table (which causes exits), and then construct shadow
page table accordingly which setups GVA->HPA mapping. It's the shadow page
table which is actually walked by the CPU when the guest is running.
W/ EPT, CPU has the concept of GVA->GPA->HPA. So literally the P2M table is
maintained as EPT table, and then CPU will first walk guest page table to
acquire GPA and then walk EPT table to get final HPA.
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