Re: [PATCH v2 08/25] x86/virt/tdx: Add SEAMCALL wrappers for TDX page cache management

[Huang, Kai]

On 1/11/2024 7:57 am, Edgecombe, Rick P wrote:
> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> index bad83f6a3b0c..bb7cdb867581 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.c
> +++ b/arch/x86/virt/vmx/tdx/tdx.c

...

> +static void tdx_clear_page(unsigned long page_pa)
> +{
> +       const void *zero_page = (const void *) __va(page_to_phys(ZERO_PAGE(0)));
> +       void *page = __va(page_pa);
> +       unsigned long i;
> +
> +       /*
> +        * The page could have been poisoned.  MOVDIR64B also clears
> +        * the poison bit so the kernel can safely use the page again.
> +        */
> +       for (i = 0; i < PAGE_SIZE; i += 64)
> +               movdir64b(page + i, zero_page);
> +       /*
> +        * MOVDIR64B store uses WC buffer.  Prevent following memory reads
> +        * from seeing potentially poisoned cache.
> +        */
> +       __mb();
> +}

Just FYI there's already one reset_tdx_pages() doing the same thing in 
x86 tdx.c:

/*
 * Convert TDX private pages back to normal by using MOVDIR64B to
 * clear these pages.  Note this function doesn't flush cache of
 * these TDX private pages.  The caller should make sure of that.
 */
static void reset_tdx_pages(unsigned long base, unsigned long size)
{
        const void *zero_page = (const void *)page_address(ZERO_PAGE(0));
        unsigned long phys, end;

        end = base + size;
        for (phys = base; phys < end; phys += 64)
                movdir64b(__va(phys), zero_page);

        /*
         * MOVDIR64B uses WC protocol.  Use memory barrier to
         * make sure any later user of these pages sees the
         * updated data.
         */
        mb();
}