question about syscall interception

question about syscall interception

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Hello everyone,

I have a few questions about syscall interception in Xen. Thanks for any
advice/suggestion.

Setting: CPU, Xen and PV Linux guest are all 64-bit

(1) If a guest app uses "syscall" instruction to launch a system call to
the guest kernel, is that behavior intercepted by Xen by default? If yes,
could someone please point me to the related code in Xen for this
interception? (anything to do with "switch_to_kernel in
xen/arch/x86/x86_64/entry.S"?) If no, then how could I intercept that
instruction in this case?

(2) If a guest app uses "int 0x80" to launch a system call, then is it true
that after "init_int80_direct_trap," the int 0x80 is no longer intercepted
by Xen, and the app can trap directly into the guest kernel? My
understanding is that "init_int80_direct_trap" initializes vcpu's
arch_vpuc.int80_bounce. Could someone please briefly explain how things
work after this struct is properly setup? How could I intercept system
calls launched in this way in Xen?

Thanks.

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