On 20 July 2018 at 00:59, Jann Horn [off-list ref] wrote:
On Thu, Jul 19, 2018 at 11:40 PM Ahmed Abd El Mawgood
Why are you implementing this in the kernel, instead of doing it in
host userspace?
I thought about implementing it completely in QEMU but It won't be
possible for few reasons:
- After talking to QEMU folks I came up to conclusion that it when it
comes to managing memory allocated for guest, it is always better to let
KVM handles everything, unless there is a good reason to play with that
memory chunk inside QEMU itself.
- But actually there is a good reason for implementing ROE in kernel space,
it is that ROE is architecture dependent to great extent. I should have
emphasized that the only currently supported architecture is X86. I am
not sure how deep the dependency on architecture goes. But as for now
the current set of patches does a SPTE enumeration as part of the process.
To my best knowledge, this isn't exposed outside arch/x68/kvm let alone
having a host user space interface for it. Also the way I am planning to
protect TLB from malicious gva -> gpa mapping is by knowing that in x86
it is possible to VMEXIT on page faults, I am not sure if it will safe to
assume that all kvm supported architectures will behave this way.
For these reasons I thought it will be better if arch dependent stuff (the
mechanism implementation) is kept in arch/*/kvm folder and with minimal
modifications to virt/kvm/* after setting a kconfig variable to enable ROE.
But I left room for the user space app using kvm to decide the rightful policy
for handling ROE violations. The way it works by KVM_EXIT_MMIO error to user
space, keeping all the architectural details hidden away from user space.
A last note is that I didn't create this from scratch, instead I extended
KVM_MEM_READONLY implementation to also allow R/O per page instead
R/O per whole slot which is already done in kernel space.