Re: [PATCH 5/9] Drivers: hv: vmbus: Support >64 VPs for a fully enlightened TDX/SNP VM

[Tianyu Lan <ltykernel@gmail.com>]

On 8/12/2023 6:18 AM, Dexuan Cui wrote:
> Don't set *this_cpu_ptr(hyperv_pcpu_input_arg) before the function
> set_memory_decrypted() returns, otherwise we run into this ticky issue:
> 
> For a fully enlightened TDX/SNP VM, in hv_common_cpu_init(),
> *this_cpu_ptr(hyperv_pcpu_input_arg) is an encrypted page before
> the set_memory_decrypted() returns.
> 
> When such a VM has more than 64 VPs, if the hyperv_pcpu_input_arg is not
> NULL, hv_common_cpu_init() -> set_memory_decrypted() -> ... ->
> cpa_flush() -> on_each_cpu() -> ... -> hv_send_ipi_mask() -> ... ->
> __send_ipi_mask_ex() tries to call hv_do_fast_hypercall16() with the
> hyperv_pcpu_input_arg as the hypercall input page, which must be a
> decrypted page in such a VM, but the page is still encrypted at this
> point, and a fatal fault is triggered.
> 
> Fix the issue by setting *this_cpu_ptr(hyperv_pcpu_input_arg) after
> set_memory_decrypted(): if the hyperv_pcpu_input_arg is NULL,
> __send_ipi_mask_ex() returns HV_STATUS_INVALID_PARAMETER immediately,
> and hv_send_ipi_mask() falls back to orig_apic.send_IPI_mask(),
> which can use x2apic_send_IPI_all(), which may be slightly slower than
> the hypercall but still works correctly in such a VM.
>
> Signed-off-by: Dexuan Cui <decui@microsoft.com>

Reviewed-by: Tianyu Lan <tiala@microsoft.com>