Re: [PATCH v2 0/7] target/i386/mshv: use hv_vp_register_page for fast register access

[Magnus Kulke <magnuskulke@linux.microsoft.com>]

On Tue, May 05, 2026 at 09:50:21PM +0300, Doru Blânzeanu wrote:
> This series adds support for using the hypervisor's vp register page
> in the mshv accelerator to optimize vcpu register access on mmio and pio
> exits.
> 
> Currently, all register reads and write go through hypercalls (ioctls),
> which adds overhead on every VM exit. The VP register page is a shared
> memory page that the hypervisor populates with vcpu register state,
> allowing Qemu to read and write registers directly without hypercalls.
> 
> The series is structured as follows:
> 1. Remove the duplicate `fetch_guest_state` function, consolidating
>   register loading into `mshv_load_regs`.
> 2. Move `mshv_arch_init_vcpu` after vcpu creation so the vcpu fd is
>   valid when we need it for mmap.
> 3. Define the `hv_vp_register_page` structure in `hvgdk_mini.h`, matching
>   the layout used by the Linux kernel's mshv driver.
> 4. Set up the register page by mmapping the vcpu fd at init time. If the
>   mmap fails, we fall back gracefully to the existing hypercall path.

I think we don't have to fallback gracefully here. If the hypervisor
doesn't support the register page, it's likely that other assumption
don't hold either. I would suggest to abort here and relax that
constraint later, if we encounter a legit scenario where mmaping the
register page would fail.

> 5. Use the register page to read registers on VM exit. General purpose
>   registers, RIP, RFLAGS, segment registers, and control registers
>   (CR0, CR4, CR4, CR8, EFER) are read directly from the page. Registers
>   not present on the page (TR, LDTR, GDTR, IDTR, CR2, APIC_BASE) are still
>   fetched via hypercall.
> 6. Use register page to write registers on vmentry. GP registers,
>   RIP, and RFLAGS are written to the page with the appropriate dirty
>   bits set, avoiding the hypercall for the standard register store.
> 
> The register page is only used when it has been successfully mmapped and
> the hypervisor has marked it as valid (`isvalid != 0`). Otherwise, the
> existing hypercall-based path is used as a fallback.
> 
> Changes since v1:
> - move hv_register_page struct definition to hvhdk.h
> - add a compile time guard around regs_page in CPUArchState
> - modify mshv_get_special_regs_vp_page to only retrieve the special
>   registers present in the register page (removed TR, LDTR, GDTR, IDTR,
>   CR2, APIC_BASE)
>   In local testing this hasn't created any regressions, and it is unlikely
>   that the mmio operations need this registers.
>   We'll want to keep an eye on this in case there are decoded operations
>   that rely on fetching these registers on every VM exit.
> - add commit to fix handle_pio_non_str and handle_pio_str to correctly
>   store modified registers back to the register page after the pio
>   operation, and clear the cpu->accel->dirty flag to avoid the
>   mshv_arch_put_registers from resetting some registers state (fpu).
> - modified register page setup to signal an error instead of a warning
>   in case mmap fails.
>   I am not sure aborting here is fine because it would make some of the
>   fallback logic redundant, and I think that's a bigger refactoring.
> 
> Doru Blânzeanu (7):
>   target/i386/mshv: remove duplicate function for reading vcpu registers
>   accel/mshv: move vcpu arch specific initialization after vcpu creation
>   include/hw/hyperv: add hv_vp_register_page struct definition
>   target/i386/mshv: hv_vp_register_page setup for the vcpu
>   target/i386/mshv: use the register page to get registers
>   target/i386/mshv: use the register page to set registers
>   target/i386/mshv: fix pio handlers clobbering device-modified
>     registers
> 
>  accel/mshv/mshv-all.c       |   3 +-
>  include/hw/hyperv/hvgdk.h   |   2 +
>  include/hw/hyperv/hvhdk.h   | 105 ++++++++++++++
>  target/i386/cpu.h           |   5 +
>  target/i386/mshv/mshv-cpu.c | 269 ++++++++++++++++++++++++++++--------
>  5 files changed, 327 insertions(+), 57 deletions(-)
> 
> -- 
> 2.53.0