Re: [PATCH v3 2/6] KVM: x86: add force_intercept_exceptions_mask

On Thu, 2021-09-02 at 16:56 +0000, Sean Christopherson wrote:

Assuming this hasn't been abandoned...

On Wed, Aug 11, 2021, Maxim Levitsky wrote:

quoted

This parameter will be used by VMX and SVM code to force
interception of a set of exceptions, given by a bitmask
for guest debug and/or kvm debug.

This is based on an idea first shown here:
https://patchwork.kernel.org/project/kvm/patch/20160301192822.GD22677@pd.tnic/

CC: Borislav Petkov <redacted>
Signed-off-by: Maxim Levitsky <redacted>
---
 arch/x86/kvm/x86.c | 3 +++
 arch/x86/kvm/x86.h | 2 ++
 2 files changed, 5 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fdc0c18339fb..092e2fad3c0d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c

@@ -184,6 +184,9 @@ module_param(force_emulation_prefix, bool, S_IRUGO);
 int __read_mostly pi_inject_timer = -1;
 module_param(pi_inject_timer, bint, S_IRUGO | S_IWUSR);
 
+uint force_intercept_exceptions_mask;
+module_param(force_intercept_exceptions_mask, uint, S_IRUGO | S_IWUSR);

Use octal permissions.  This also can't be a simple writable param, at least not
without a well-documented disclaimer, as there's no guarantee a vCPU will update
its exception bitmap in a timely fashion.  An alternative to a module param would
be to extend/add a per-VM ioctl(), e.g. maybe KVM_SET_GUEST_DEBUG?  The downside
of an ioctl() is that it would require userspace enabling :-/

All other module params in this file use macros for permissions, that is why
I used them too.

I'll add a comment with a disclaimer here - this is only for debug.
I strongly don't want to have this as ioctl as that will indeed need qemu patches,
not to mention things like unit tests and which don't even always use qemu.

Or I can make this parameter read-only. I don't mind reloading kvm module when
I change this parameter.


Best regards,
	Maxim Levitsky

PS: Forgot to send this mail, it was in my drafts folder.