From: Nuno Das Neves <redacted> Sent: Monday, March 10, 2025 2:47 PM

On 3/7/2025 9:38 AM, Michael Kelley wrote:

quoted

From: Nuno Das Neves <redacted> Sent: Friday, February 28,

2025 4:38 PM

quoted

quoted

On 2/26/2025 3:43 PM, Stanislav Kinsburskii wrote:

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On Wed, Feb 26, 2025 at 03:08:02PM -0800, Nuno Das Neves wrote:

quoted

This will handle SYNIC interrupts such as intercepts, doorbells, and
scheduling messages intended for the mshv driver.

Signed-off-by: Nuno Das Neves <redacted>
Reviewed-by: Wei Liu <wei.liu@kernel.org>
Reviewed-by: Tianyu Lan <redacted>
---
 arch/x86/kernel/cpu/mshyperv.c | 9 +++++++++
 drivers/hv/hv_common.c         | 5 +++++
 include/asm-generic/mshyperv.h | 1 +
 3 files changed, 15 insertions(+)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 0116d0e96ef9..616e9a5d77b4 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c

@@ -107,6 +107,7 @@ void hv_set_msr(unsigned int reg, u64 value)
 }
 EXPORT_SYMBOL_GPL(hv_set_msr);

+static void (*mshv_handler)(void);
 static void (*vmbus_handler)(void);
 static void (*hv_stimer0_handler)(void);
 static void (*hv_kexec_handler)(void);

@@ -117,6 +118,9 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_hyperv_callback)
 	struct pt_regs *old_regs = set_irq_regs(regs);

 	inc_irq_stat(irq_hv_callback_count);
+	if (mshv_handler)
+		mshv_handler();

Can mshv_handler be defined as a weak symbol doing nothing instead
of defining it a null pointer?
This should allow to simplify this code and get rid of
hv_setup_mshv_handler, which looks redundant.

Interesting, I tested this and it does seems to work! It seems like
a good change, thanks.

Just be a bit careful. When CONFIG_HYPERV=n, mshyperv.c still gets
built even through none of the other Hyper-V related files do.  There
are #ifdef CONFIG_HYPERV in mshyperv.c to eliminate references to
Hyper-V files that wouldn't be built. I'd suggest doing a test build with
that configuration to make sure it's all clean.

Thanks Michael - I don't think it would be an issue since the __weak version
would be defined in mshyperv.c itself, replacing the function pointer.

Yes, sounds right to me.

However, I went and tested this __weak version again with CONFIG_MSHV_ROOT=m
and it does not actually work. Everything seems ok at first (it compiles,
can insert the module), but upon starting a guest, the interrupts don't get
delivered to the root (or rather, they don't get handled by mshv_hander()).

This seems to match with what the ld docs say - There's an option
LD_DYNAMIC_LINK to allow __weak symbols to be overridden by the dynamic
linker, but this is not enabled in the kernel.

Yeah, I recall learning the hard way that a symbol defined in a module doesn't
override a __weak symbol in the kernel image. At the time, I gave up and took
a different path, and didn't get as far as looking at 'ld' options like you did. :-)

Michael