Re: [PATCHv6 10/16] x86/tdx: Convert shared memory back to private on kexec

["Kalra, Ashish" <ashish.kalra@amd.com>]

Hello Kirill,

On 1/24/2024 6:55 AM, Kirill A. Shutemov wrote:
> TDX guests allocate shared buffers to perform I/O. It is done by
> allocating pages normally from the buddy allocator and converting them
> to shared with set_memory_decrypted().
>
> The second kernel has no idea what memory is converted this way. It only
> sees E820_TYPE_RAM.
>
> Accessing shared memory via private mapping is fatal. It leads to
> unrecoverable TD exit.
>
> On kexec walk direct mapping and convert all shared memory back to
> private. It makes all RAM private again and second kernel may use it
> normally.
>
> The conversion occurs in two steps: stopping new conversions and
> unsharing all memory. In the case of normal kexec, the stopping of
> conversions takes place while scheduling is still functioning. This
> allows for waiting until any ongoing conversions are finished. The
> second step is carried out when all CPUs except one are inactive and
> interrupts are disabled. This prevents any conflicts with code that may
> access shared memory.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> ---
>   arch/x86/coco/tdx/tdx.c | 124 +++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 122 insertions(+), 2 deletions(-)
<snip>
> +static void tdx_kexec_stop_conversion(bool crash)
> +{
> +	/* Stop new private<->shared conversions */
> +	conversion_allowed = false;
> +
> +	/*
> +	 * Make sure conversion_allowed is cleared before checking
> +	 * conversions_in_progress.
> +	 */
> +	barrier();
> +
> +	/*
> +	 * Crash kernel reaches here with interrupts disabled: can't wait for
> +	 * conversions to finish.
> +	 *
> +	 * If race happened, just report and proceed.
> +	 */
> +	if (!crash) {
> +		unsigned long timeout;
> +
> +		/*
> +		 * Wait for in-flight conversions to complete.
> +		 *
> +		 * Do not wait more than 30 seconds.
> +		 */
> +		timeout = 30 * USEC_PER_SEC;
> +		while (atomic_read(&conversions_in_progress) && timeout--)
> +			udelay(1);
> +	}
> +
> +	if (atomic_read(&conversions_in_progress))
> +		pr_warn("Failed to finish shared<->private conversions\n");
> +}
> +
> +static void tdx_kexec_unshare_mem(void)
> +{
> +	unsigned long addr, end;
> +	long found = 0, shared;
> +
> +	/*
> +	 * Walk direct mapping and convert all shared memory back to private,
> +	 */
> +
> +	addr = PAGE_OFFSET;
> +	end  = PAGE_OFFSET + get_max_mapped();
> +
> +	while (addr < end) {
> +		unsigned long size;
> +		unsigned int level;
> +		pte_t *pte;
> +
> +		pte = lookup_address(addr, &level);
> +		size = page_level_size(level);
> +
> +		if (pte && pte_decrypted(*pte)) {
> +			int pages = size / PAGE_SIZE;
> +
> +			/*
> +			 * Touching memory with shared bit set triggers implicit
> +			 * conversion to shared.
> +			 *
> +			 * Make sure nobody touches the shared range from
> +			 * now on.
> +			 */
> +			set_pte(pte, __pte(0));
> +
> +			if (!tdx_enc_status_changed(addr, pages, true)) {
> +				pr_err("Failed to unshare range %#lx-%#lx\n",
> +				       addr, addr + size);
> +			}
> +
> +			found += pages;
> +		}
> +
> +		addr += size;
> +	}
> +
> +	__flush_tlb_all();
> +
> +	shared = atomic_long_read(&nr_shared);
> +	if (shared != found) {
> +		pr_err("shared page accounting is off\n");
> +		pr_err("nr_shared = %ld, nr_found = %ld\n", shared, found);
> +	}
> +}
In case of SNP and crash/kdump case, we need to prevent the boot_ghcb 
being converted to shared (in snp_kexec_unshare_mem()) as the boot_ghcb 
is required to handle all I/O for disabling IO-APIC/lapic, hpet, etc., 
as the enc_kexec_unshare_mem() callback is invoked before the apics, 
hpet, etc. are disabled.

Is there any reason why enc_kexec_unshare_mem() callback is invoked in 
crash case before the IO-APIC/lapic, hpet, etc. are shutdown/disabled ?

In case of kexec, enc_kexec_unshare_mem() callback is invoked after the 
IO-APIC/lapic, hpet, iommu, etc. have already been disabled/shutdown, 
hence, this callback can transition all guest shared memory (including 
boot_ghcb) back to private.

Thanks, Ashish